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/Teams.cs

@@ -0,0 +1,79 @@
+namespace DodoSSH.Domain;
+
+/// <summary>
+/// A group of users who can share vaults.
+/// </summary>
+/// <remarks>
+/// The tables exist from the first migration although team features ship in M3. Adding them
+/// later would mean altering <see cref="Vault"/> to introduce a foreign key on a live table, and
+/// the cost of carrying two unused tables is far lower than that.
+/// </remarks>
+public sealed class Team
+{
+    /// <summary>Primary key.</summary>
+    public Guid Id { get; set; }
+
+    /// <summary>Display name.</summary>
+    public string Name { get; set; } = string.Empty;
+
+    /// <summary>URL-safe unique identifier.</summary>
+    public string Slug { get; set; } = string.Empty;
+
+    /// <summary>Optional description.</summary>
+    public string? Description { get; set; }
+
+    /// <summary>Who created it.</summary>
+    public Guid CreatedByUserId { get; set; }
+
+    /// <summary>Creation timestamp.</summary>
+    public DateTimeOffset CreatedAtUtc { get; set; }
+
+    /// <summary>Soft-delete marker.</summary>
+    public DateTimeOffset? DeletedAtUtc { get; set; }
+
+    /// <summary>Members.</summary>
+    public ICollection<TeamMembership> Memberships { get; } = [];
+}
+
+/// <summary>
+/// A user's membership of a team.
+/// </summary>
+/// <remarks>
+/// Revoked memberships are retained rather than deleted, so historic audit entries remain
+/// resolvable to a person.
+/// </remarks>
+public sealed class TeamMembership
+{
+    /// <summary>Primary key.</summary>
+    public Guid Id { get; set; }
+
+    /// <summary>The team.</summary>
+    public Guid TeamId { get; set; }
+
+    /// <summary>The team.</summary>
+    public Team? Team { get; set; }
+
+    /// <summary>The member.</summary>
+    public Guid UserId { get; set; }
+
+    /// <summary>The member.</summary>
+    public UserAccount? User { get; set; }
+
+    /// <summary>Role within the team.</summary>
+    public TeamRole Role { get; set; }
+
+    /// <summary>Membership state.</summary>
+    public MembershipStatus Status { get; set; }
+
+    /// <summary>Who invited them.</summary>
+    public Guid? InvitedByUserId { get; set; }
+
+    /// <summary>When the invitation was accepted.</summary>
+    public DateTimeOffset? JoinedAtUtc { get; set; }
+
+    /// <summary>Creation timestamp.</summary>
+    public DateTimeOffset CreatedAtUtc { get; set; }
+
+    /// <summary>Soft-delete marker.</summary>
+    public DateTimeOffset? DeletedAtUtc { get; set; }
+}