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/XminConcurrency.cs

@@ -0,0 +1,46 @@
+using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore.Metadata.Builders;
+
+namespace DodoSSH.Infrastructure;
+
+/// <summary>
+/// Maps PostgreSQL's <c>xmin</c> system column as an optimistic concurrency token.
+/// </summary>
+/// <remarks>
+/// <para>
+/// Npgsql's <c>UseXminAsConcurrencyToken</c> helper no longer exists in EF 10, so the shadow
+/// property is configured directly here rather than repeated in every entity configuration.
+/// </para>
+/// <para>
+/// <c>xmin</c> is a system column that PostgreSQL maintains, so it must never appear in a
+/// <c>CREATE TABLE</c>. That is what <see cref="RelationalPropertyBuilderExtensions.HasColumnName"/>
+/// combined with <c>ValueGeneratedOnAddOrUpdate</c> achieves; an
+/// <c>Infrastructure</c> test asserts the generated DDL does not declare it.
+/// </para>
+/// <para>
+/// This token is strictly internal. It is never exposed to clients: <c>xmin</c> is not stable
+/// across <c>VACUUM FREEZE</c>, so using it as a sync cursor would silently break. Client-visible
+/// versioning is the separate monotonic <c>Version</c> column on item rows.
+/// </para>
+/// </remarks>
+public static class XminConcurrency
+{
+    /// <summary>Name of the shadow property and of the PostgreSQL system column.</summary>
+    public const string PropertyName = "xmin";
+
+    /// <summary>Configures <c>xmin</c> as this entity's concurrency token.</summary>
+    public static EntityTypeBuilder<TEntity> UseXminConcurrencyToken<TEntity>(
+        this EntityTypeBuilder<TEntity> builder)
+        where TEntity : class
+    {
+        ArgumentNullException.ThrowIfNull(builder);
+
+        builder.Property<uint>(PropertyName)
+            .HasColumnName(PropertyName)
+            .HasColumnType("xid")
+            .ValueGeneratedOnAddOrUpdate()
+            .IsConcurrencyToken();
+
+        return builder;
+    }
+}