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.

tests/DodoSSH.Infrastructure.Tests/PostgresFixture.cs

@@ -0,0 +1,58 @@
+using DodoSSH.Infrastructure;
+using Microsoft.EntityFrameworkCore;
+using Testcontainers.PostgreSql;
+using Xunit;
+
+namespace DodoSSH.Infrastructure.Tests;
+
+/// <summary>
+/// One PostgreSQL container per test assembly, migrated once.
+/// </summary>
+/// <remarks>
+/// A container per test would dominate the runtime. Tests that write must therefore use distinct
+/// identifiers rather than assuming an empty database.
+/// </remarks>
+public sealed class PostgresFixture : IAsyncLifetime
+{
+    private readonly PostgreSqlContainer container = new PostgreSqlBuilder()
+        .WithImage("postgres:18-alpine")
+        .WithDatabase("dodossh")
+        .WithUsername("postgres")
+        .WithPassword("test")
+        .Build();
+
+    /// <summary>Connection string for the running container.</summary>
+    public string ConnectionString => container.GetConnectionString();
+
+    /// <inheritdoc />
+    public async ValueTask InitializeAsync()
+    {
+        await container.StartAsync();
+
+        await using var context = CreateContext();
+        await context.Database.MigrateAsync();
+    }
+
+    /// <inheritdoc />
+    public async ValueTask DisposeAsync() => await container.DisposeAsync();
+
+    /// <summary>Creates a context against the container.</summary>
+    public DodoDbContext CreateContext()
+    {
+        var options = new DbContextOptionsBuilder<DodoDbContext>()
+            .UseNpgsql(ConnectionString, npgsql =>
+                npgsql.MigrationsHistoryTable("__EFMigrationsHistory", DodoDbContext.SchemaName))
+            .UseSnakeCaseNamingConvention()
+            .Options;
+
+        return new DodoDbContext(options);
+    }
+}
+
+/// <summary>Shares one container across every test class in the assembly.</summary>
+[CollectionDefinition(Name)]
+public sealed class PostgresCollection : ICollectionFixture<PostgresFixture>
+{
+    /// <summary>Collection name.</summary>
+    public const string Name = "postgres";
+}