Spelunk: sign each device in through the browser instead of keeping Spelunk API keys in the vault

A Spelunk API key reaches one organisation, so a server elsewhere - oc-linux in Orange Climate - was
refused before it reached the agent. Each device now signs in to a Spelunk once, under Settings ->
Security -> Spelunk or from a Spelunk host's editor: the OAuth authorization code flow with PKCE for a
native app (RFC 8252), with Spelunk as the authorization server. The browser opens Spelunk's
/api/app-sign-in/authorize with a loopback redirect (the app's own scheme on Android), the person signs
in there however they normally do and allows "DodoSSH on <device>", and the code is redeemed at
/api/app-sign-in/token for an app session token. That token opens terminals in every organisation its
person is in, with the role they have on each server at the time; tunnel tickets send no organisation.

The sign-in stays on the device: sealed in the local cache under the LocalCacheKey (AAD resource type
18, bound to its row), never synced, wiped with the cache, and only ever sent to the Spelunk it came
from. A 401 from Spelunk forgets it; SIGN OUT ends it there and here, and so does signing out of
DodoSSH. Before every Spelunk connection DodoSSH asks its own identity provider whether its sign-in
still stands (a forced refresh, at most once a minute), and an account it refuses is signed out of
DodoSSH and of every Spelunk on the device.

A route names no key any more: Spelunk hosts are written at host schema 11, which an older build shows
read-only and asks to be updated for, and a payload that still names a key credential decodes with the
id skipped. The SpelunkKey item type is gone: SyncEntityType 14, ChangeEntityType 14 and AAD type 17
are retired, the server drops spelunk_key and its change-log rows, and the client cache drops its
cached items, outbox entries and conflicts of that type. A link still never starts a sign-in: from a
Spelunk this device is not signed in to it opens the editor, which offers to.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/crypto.md

@@ -300,7 +300,7 @@ value can forge a field boundary. UUIDs must be serialised in RFC 4122 order —
 `1` User, `2` Device, `3` Vault, `4` Host, `5` Credential, `6` SshKey, `7` HostGroup,
 `8` Tag, `9` Snippet, `10` PortForward, `11` KnownHostKey, `12` HostTag,
 `13` HostCredential, `14` ConnectionLogEntry, `15` ActivityLogEntry, `16` ObjectStore,
-`17` SpelunkKey.
+`18` SpelunkSignIn. `17` is retired and never reused.
 
 > **Added 2026-07-29:** `12` and `13`. `Contracts.SyncEntityType` has listed `HostTag` and
 > `HostCredential` as syncable since the contract was frozen, but this table had no value for
@@ -319,8 +319,12 @@ value can forge a field boundary. UUIDs must be serialised in RFC 4122 order —
 > `ConnectionLogEntry` is `11` there and `14` here. Nothing casts between them, and
 > `AadResourceTypeTests` pins each pairing by name for exactly that reason.
 
-> **Added 2026-09-29:** `17` SpelunkKey, a Spelunk API key and the address it may be sent to. It is
-> `14` on the wire, three behind, as `16` ObjectStore is `13`.
+> **Added 2026-09-29, removed 2026-10-07:** `17` SpelunkKey, a Spelunk API key and the address it
+> may be sent to (`14` on the wire). Removed in beta when DodoSSH started signing in to Spelunk instead;
+> both numbers stay retired.
+
+> **Added 2026-10-07:** `18` SpelunkSignIn, a device's sign-in to one Spelunk, sealed in the local cache
+> (§4.2's LocalCache AAD) and bound to its row. It is never synced and has no `SyncEntityType`.
 
 `0` means not applicable and is legal only where the table in §4.2 implies no resource.