Add data model, DbContext and initial migration (M1)

Schema for identity, vaults, grants, hosts and the sync change log, verified against a
real PostgreSQL 18 container rather than an in-memory provider: partial unique indexes,
CHECK constraints, citext and identity-always columns are all provider behaviour that an
in-memory fake would not exercise.

Invariants pushed into the database, so they hold even when application code has a bug:
- ck_host_relay_target is a security boundary, not tidiness. A host may carry a plaintext
  hostname and port ONLY when relay is deliberately enabled. Both directions are tested;
  the important one is that relay-disabled hosts cannot carry an address, since otherwise
  a bug would silently give the server infrastructure visibility it was never granted.
- ck_vault_owner: exactly one of owner_user_id or team_id, or permission resolution would
  have no defined answer.
- ck_vault_key_grant_recipient: member grants name a user; recovery and escrow grants are
  wrapped to a key and must not.
- ck_user_key_wrap_kdf: a password-derived wrap without its parameters is permanently
  unopenable, so a partial write is rejected outright.

Present from the first migration on purpose:
- GrantKind (Member/Recovery/Escrow). Recovery cannot be bolted on later — every vault
  created before it existed would be unrecoverable by design.
- team and team_membership, though team features are M3. Adding them later would mean
  introducing a foreign key on a live vault table.
- Host.ContentKeyId, reserved for per-item content keys wrapped to individual users.
- user_key as its own table, so key rotation does not require altering the user row.

Two things verified rather than assumed:
- Npgsql's UseXminAsConcurrencyToken helper no longer exists in EF 10, so xmin is mapped
  directly in XminConcurrency. The generated migration *looks* like it creates an xmin
  column; it does not. Confirmed by inspecting pg_attribute (attnum -2, a system column)
  and by grepping the emitted DDL. A test pins both, because had it created a real column
  PostgreSQL would have rejected the name.
- EF Core is now pinned centrally. The Npgsql provider asks for 10.0.4 while
  EntityFrameworkCore.Design pulls 10.0.10, and because Design is PrivateAssets=all that
  higher version does not flow to referencing projects — producing a CS1705 in any test
  project referencing Infrastructure.

Also commits artifacts/schema/v0.1.sql, the idempotent script, as the baseline for future
upgrade tests.

Verified: 0 warnings, 122 tests pass (27 new against Postgres), format clean.

src/DodoSSH.Domain/Hosts.cs

@@ -0,0 +1,89 @@
+namespace DodoSSH.Domain;
+
+/// <summary>
+/// An SSH host.
+/// </summary>
+/// <remarks>
+/// <para>
+/// The only vault item type in M1. Everything sensitive — username, notes, jump chain, SSH
+/// options — lives inside <see cref="Payload"/>. There is deliberately no plaintext label:
+/// access-control administration happens in the client, which can decrypt names, so the server
+/// never needs a searchable title.
+/// </para>
+/// <para>
+/// <see cref="Hostname"/> and <see cref="Port"/> are the one deliberate plaintext concession, and
+/// only when <see cref="RelayEnabled"/> is set. The relay must resolve its target server-side or
+/// it becomes an authenticated open TCP proxy into the operator's own network. A database CHECK
+/// constraint enforces the pairing so it cannot drift. See ADR 0004.
+/// </para>
+/// </remarks>
+public sealed class Host
+{
+    /// <summary>Primary key. UUIDv7, generated by the client so items can be created offline.</summary>
+    public Guid Id { get; set; }
+
+    /// <summary>Owning vault.</summary>
+    public Guid VaultId { get; set; }
+
+    /// <summary>Owning vault.</summary>
+    public Vault? Vault { get; set; }
+
+    /// <summary>The encrypted item: a DSH1 envelope. Opaque to the server.</summary>
+    public byte[] Payload { get; set; } = [];
+
+    /// <summary>The item's data key, wrapped under the vault key. Opaque.</summary>
+    public byte[]? DataKeyWrap { get; set; }
+
+    /// <summary>
+    /// Reserved for per-item content keys wrapped to individual users, which is what will make
+    /// per-item access control cryptographic rather than server-enforced. Present from the first
+    /// migration so that lands without a migration; see docs/crypto.md §3.
+    /// </summary>
+    public Guid? ContentKeyId { get; set; }
+
+    /// <summary>Vault key generation this payload was encrypted under.</summary>
+    public int KeyGeneration { get; set; }
+
+    /// <summary>AAD rule version, enabling a lazy re-encrypt-on-write migration later.</summary>
+    public short PayloadAadVersion { get; set; }
+
+    /// <summary>Whether this host may be dialled through the server relay.</summary>
+    public bool RelayEnabled { get; set; }
+
+    /// <summary>Target hostname. Permitted only when <see cref="RelayEnabled"/> is set.</summary>
+    public string? Hostname { get; set; }
+
+    /// <summary>Target port. Permitted only when <see cref="RelayEnabled"/> is set.</summary>
+    public int? Port { get; set; }
+
+    /// <summary>Owning group, for tree placement. Groups arrive in M2.</summary>
+    public Guid? GroupId { get; set; }
+
+    /// <summary>
+    /// Client-visible, monotonic item version. Used for optimistic concurrency on push, and
+    /// deliberately distinct from the internal <c>xmin</c> guard, which is never exposed because
+    /// it is not stable across VACUUM FREEZE.
+    /// </summary>
+    public int Version { get; set; }
+
+    /// <summary>Latest change-log sequence touching this row, so a delta pull can join directly.</summary>
+    public long ChangeSequence { get; set; }
+
+    /// <summary>Creation timestamp.</summary>
+    public DateTimeOffset CreatedAtUtc { get; set; }
+
+    /// <summary>Last modification timestamp.</summary>
+    public DateTimeOffset UpdatedAtUtc { get; set; }
+
+    /// <summary>
+    /// Soft-delete marker. Deletes are tombstones: a client that has been offline must be able to
+    /// learn an item went away, and a vanished row is indistinguishable from one never seen.
+    /// </summary>
+    public DateTimeOffset? DeletedAtUtc { get; set; }
+
+    /// <summary>Who created it.</summary>
+    public Guid CreatedByUserId { get; set; }
+
+    /// <summary>Who last modified it.</summary>
+    public Guid UpdatedByUserId { get; set; }
+}