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.Infrastructure/DodoDbContext.cs

@@ -0,0 +1,75 @@
+using DodoSSH.Domain;
+using Microsoft.EntityFrameworkCore;
+
+namespace DodoSSH.Infrastructure;
+
+/// <summary>
+/// The application database context.
+/// </summary>
+/// <remarks>
+/// <para>
+/// Everything lives in the <c>dodo</c> schema with snake_case names. Timestamps are
+/// <c>timestamptz</c> and always UTC, so there is no tzdata dependency and no conflict with
+/// <c>InvariantGlobalization</c>.
+/// </para>
+/// <para>
+/// Two concurrency mechanisms coexist deliberately. <c>Version</c> on item rows is the
+/// client-visible, monotonic value used for <c>expectedVersion</c> conflict detection.
+/// <c>xmin</c> is the server-side optimistic guard and is never exposed, because it is not stable
+/// across <c>VACUUM FREEZE</c> and must never become a client cursor.
+/// </para>
+/// </remarks>
+public class DodoDbContext(DbContextOptions<DodoDbContext> options) : DbContext(options)
+{
+    /// <summary>The database schema every table lives in.</summary>
+    public const string SchemaName = "dodo";
+
+    /// <summary>User accounts.</summary>
+    public DbSet<UserAccount> Users => Set<UserAccount>();
+
+    /// <summary>Identity key generations.</summary>
+    public DbSet<UserKey> UserKeys => Set<UserKey>();
+
+    /// <summary>Wraps of users' secret bundles.</summary>
+    public DbSet<UserKeyWrap> UserKeyWraps => Set<UserKeyWrap>();
+
+    /// <summary>Enrolled devices.</summary>
+    public DbSet<Device> Devices => Set<Device>();
+
+    /// <summary>The append-only key transparency log.</summary>
+    public DbSet<KeyLogEntry> KeyLog => Set<KeyLogEntry>();
+
+    /// <summary>Teams.</summary>
+    public DbSet<Team> Teams => Set<Team>();
+
+    /// <summary>Team memberships.</summary>
+    public DbSet<TeamMembership> TeamMemberships => Set<TeamMembership>();
+
+    /// <summary>Vaults.</summary>
+    public DbSet<Vault> Vaults => Set<Vault>();
+
+    /// <summary>Wrapped vault keys.</summary>
+    public DbSet<VaultKeyGrant> VaultKeyGrants => Set<VaultKeyGrant>();
+
+    /// <summary>SSH hosts.</summary>
+    public DbSet<Host> Hosts => Set<Host>();
+
+    /// <summary>The per-vault change log that delta sync reads.</summary>
+    public DbSet<SyncChange> SyncChanges => Set<SyncChange>();
+
+    /// <summary>Applied-operation receipts, for exactly-once retries.</summary>
+    public DbSet<SyncOperationReceipt> SyncOperationReceipts => Set<SyncOperationReceipt>();
+
+    /// <inheritdoc />
+    protected override void OnModelCreating(ModelBuilder modelBuilder)
+    {
+        ArgumentNullException.ThrowIfNull(modelBuilder);
+
+        modelBuilder.HasDefaultSchema(SchemaName);
+        modelBuilder.HasPostgresExtension("citext");
+
+        modelBuilder.ApplyConfigurationsFromAssembly(typeof(DodoDbContext).Assembly);
+
+        base.OnModelCreating(modelBuilder);
+    }
+}