Move the keys when a membership changes, not just the flag

Adding somebody to a team granted them nothing readable and removing them
rotated nothing. Both were honest — the interface said so in as many words — and
both left the actual work to a button somebody had to remember to press, on a
machine that happened to hold the key. Adding now wraps every team vault this
machine can open to the new member, and removing revokes their grants and moves
each of those vaults to a fresh key that goes to whoever is left.

The rotation is where the design had to be decided rather than written. A vault
key is per generation and an item carries the generation it was sealed under, so
advancing the vault and withdrawing the old grants would make everything already
stored unreadable to everybody, including whoever pressed the button. So earlier
grants are kept: a member holds one per generation, /me serves them as
PriorKeyWraps, and VaultKeyring holds a key per generation — the newest for
writing, the item's own for reading, chosen per item on every read path. Sharing
issues one grant per generation held, because a recipient handed only the current
key would open the vault to find most of it undecryptable; revocation takes every
generation, because leaving the history behind leaves them able to read
everything written before the rotation.

The bump itself is one server transaction. POST /vaults/{id}/rekey must name
exactly current + 1 and the vault's xmin token makes that binding, so two admins
rotating at once do not both walk away believing they succeeded — the second is
refused and told to read the vault again. The server contributes the moment and
no cryptography: it cannot generate the key, cannot tell that the one it is
handed differs from the old one, and checks that the caller held the old one the
only way it can, by requiring a live grant at the current generation.

What this does not do is re-encrypt what is already stored, and the product says
so rather than the reassuring version: everything written from the rotation
onwards is unreadable to the person who left, and nothing about the past changes.
That half is deferred and is safe to add incrementally precisely because a vault
at mixed generations stays readable. ADR 0010 records the alternatives — revoking
the old grants, chaining each key under its successor, re-sealing every item in
one request against a server that caps a push at 500 operations — and why each
was rejected.

Two things fell out of the change rather than being asked for. The grant listing
would have shown a member once per generation, so it now returns one row per
holder carrying the best key they hold, which is what makes a row below the
vault's generation mean "still owed the new key". And MarkUnreadable gives up the
write target as well as reporting: a client whose vault was rotated elsewhere
would otherwise have gone on sealing items under its superseded key — readable to
its author, unreadable to everybody else, with nothing to show for it.

docs/design-import-gaps.md

@@ -410,9 +410,11 @@ answerable by anybody willing to create a team first. It simply gets claimed soo
 | Shared vaults | server + client | A team owns vaults; each is created with the creator's own grant, because a vault with no grant is a container nobody can open. |
 | Roles | contracts + server | `TeamMemberRole` on the wire, numerically pinned to `DodoSSH.Domain.TeamRole` by a test. Viewer reads, Member writes, Admin and Owner also share and administer. |
 | Members table | server | `TeamMemberSummary`, and a directory that resolves an exact email to a public key. |
-| Sharing an item | client | `VaultSession.ShareVaultAsync`: verify the recipient's key against the key log, wrap, sign, record. The server stores the wrap and the signature and can check neither. |
+| Sharing an item | client | `VaultSession.ShareVaultAsync`: verify the recipient's key against the key log, wrap, sign, record. The server stores the wrap and the signature and can check neither. One grant per generation the sharing client holds, so a recipient can read a rotated vault's history and not only what happens next. |
+| Adding a member shares the team's vaults | client | Adding somebody wraps every team vault the adding machine can open to them, as part of the add rather than as a button to remember. Membership and a key are still two acts on two machines; the client just performs both. A vault this machine holds no key to is skipped and named. |
+| Removing a member rotates the vaults | server + client | `POST /api/v1/vaults/{id}/rekey` advances the generation and records the caller's new grant in one transaction — the server contributes the moment and no cryptography. The client then wraps the new key to the members who remain. Grants for earlier generations are kept, or the vault's stored items would become unreadable to everybody. See [ADR 0010](adr/0010-vault-key-rotation.md). |
 | Pending invites, and withdrawing one | server | A `team_invitation` row per (team, address), listed beside the members it is about and withdrawable until it is taken up. It becomes a membership when an account with that address signs in — **and only if the access token asserts `email_verified`**, because membership is authorisation and an invitation anybody could take by naming somebody else's address is a way in. Fourteen days, because an address that is reassigned would otherwise carry a standing offer to whoever holds the job next. |
-| Ownership transfer | server | `POST /api/v1/teams/{id}/owner`, owner only. One transaction: the named member becomes owner and the outgoing owner becomes an admin. Not two role changes — ownership is sole, so promoting first leaves the team owned twice and demoting first leaves it owned by nobody. The outgoing owner is demoted rather than removed, because removing them would revoke their vault key grants and flag every team vault for rekey, which is a far larger act than the one being asked for. |
+| Ownership transfer | server | `POST /api/v1/teams/{id}/owner`, owner only. One transaction: the named member becomes owner and the outgoing owner becomes an admin. Not two role changes — ownership is sole, so promoting first leaves the team owned twice and demoting first leaves it owned by nobody. The outgoing owner is demoted rather than removed, because removing them would revoke their vault key grants and rotate every team vault, which is a far larger act than the one being asked for. |
 | `LAST ACTIVE` | server | Real, and coarse on purpose. `UserAccount.LastSeenAtUtc` is now refreshed on ordinary authenticated requests, at most once per account per hour: writing it per request would put an UPDATE on the hot path of every authenticated call and start losing races on `user_account`'s own concurrency token. So the column answers "this week or not", which is the granularity the question is actually asked at, and is shown coarsely rather than to the minute. |
 | Renaming and archiving a team | server | `PUT` and `DELETE /api/v1/teams/{id}`. The slug is deliberately not renameable: it is unique only among *live* teams, so a rename could take a slug an archived team still holds and strand it. Archiving soft-deletes the team, every membership and every pending invitation in one transaction — and is refused outright while the team owns any vault. |
 
@@ -424,7 +426,7 @@ answerable by anybody willing to create a team first. It simply gets claimed soo
 | The invitation mail, and **resend** | server | An outbound mail path: an SMTP configuration, a template, a bounce story and a deliverability problem, none of which this server has. | **Nothing is sent, and the interface says so.** An invitation is a standing instruction rather than a message — the next account to sign in with that address joins the team — so there is no token, no link, and nothing to resend. Telling somebody to sign in is done over a channel this server does not carry. A link nobody can deliver would be worse than no link. |
 | Archiving a team that owns vaults | — | Nothing that would be safe. A team vault resolves through membership, so archiving would take those vaults away from everybody holding a key, silently, including the caller — and nothing in this product deletes a vault, so there is no sequence of calls that turns the refusal into a success. | Refused, with `team-not-empty` and a count of the vaults in the way. A stated limit rather than a coming feature, for the reason the SFTP layer refuses a recursive delete: a refusal is visible and a quiet removal is not. |
 | `SSO · OIDC · okta.dodotech.dev` | server | Per-team SSO. Authentication is one global JWT scheme bound to one authority. | Omitted. |
-| A rekey after a membership change | client | Re-wrapping every item's data key under a fresh vault key, which only a client holding the current one can do. M5. | The vault is flagged `RekeyRequired` and the row says a rotation is owed. |
+| Re-sealing a rotated vault's stored items | client | Re-wrapping every item's data key under the new vault key, in batches, which only a client holding both keys can do. M5. | The rotation itself ships — see the row above. Existing items keep the generation they were sealed under and stay readable, because every member keeps the keys they were granted. What is outstanding is closing the gap where a departed member's copy of the old key would still open old ciphertext they later obtained. |
 
 > **The trap this document warned about is still a trap.** `GET /api/v1/meta` advertises
 > `features: ["teams"]` *unconditionally* (`MetaEndpoints.cs`). It was meaningless when nothing implemented