feat(data): add task numbers schema, allocator, and backfill migration

TaskEntity.Number is a global, monotonically increasing, never-reused
integer (displayed as #123), allocated from AppSettingsEntity.NextTaskNumber
via a single UPDATE...RETURNING statement rather than MAX(number)+1, which
would reissue a deleted task's number. Both insert paths (TaskRepository.
AddAsync and CreateChildAsync) route through the new TaskNumberAllocator,
with a bounded retry on a unique-index collision. One migration adds the
columns, backfills existing rows in creation order, and creates the unique
index afterwards. Data-layer only; MCP/UI wiring is later slices.

tests/ClaudeDo.Data.Tests/TaskNumberMigrationTests.cs

@@ -0,0 +1,84 @@
+using ClaudeDo.Data;
+using ClaudeDo.Data.Models;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore.Infrastructure;
+using Microsoft.EntityFrameworkCore.Migrations;
+using Microsoft.Extensions.DependencyInjection;
+
+namespace ClaudeDo.Data.Tests;
+
+// EnsureCreated (used by every other Data.Tests fixture) builds the schema straight from the
+// current model and skips migrations entirely, so it can never exercise the AddTaskNumbers
+// backfill. This test runs a real Migrate() against a database seeded with pre-migration rows.
+public sealed class TaskNumberMigrationTests : IDisposable
+{
+    private const string MigrationBeforeTaskNumbers = "20260810115437_AddFailureReason";
+
+    private readonly string _dbPath;
+    private readonly DbContextOptions<ClaudeDoDbContext> _options;
+
+    public TaskNumberMigrationTests()
+    {
+        _dbPath = Path.Combine(Path.GetTempPath(), $"claudedo_tasknum_migration_{Guid.NewGuid():N}.db");
+        _options = new DbContextOptionsBuilder<ClaudeDoDbContext>()
+            .UseSqlite($"Data Source={_dbPath}")
+            .Options;
+    }
+
+    public void Dispose()
+    {
+        foreach (var suffix in new[] { "", "-wal", "-shm" })
+            try { File.Delete(_dbPath + suffix); } catch { }
+    }
+
+    [Fact]
+    public async Task Migrate_backfills_task_numbers_in_creation_order_with_id_tiebreak()
+    {
+        await using (var ctx = new ClaudeDoDbContext(_options))
+        {
+            var migrator = ctx.Database.GetInfrastructure().GetRequiredService<IMigrator>();
+            migrator.Migrate(MigrationBeforeTaskNumbers);
+
+            var conn = ctx.Database.GetDbConnection();
+            await conn.OpenAsync();
+
+            await ExecAsync(conn, """
+                INSERT INTO lists (id, name, created_at) VALUES ('l1', 'List', '2026-01-01 00:00:00.0000000');
+                """);
+
+            // Inserted out of creation order, and 'd'/'c' share a timestamp to exercise the id tiebreak.
+            await ExecAsync(conn, InsertTaskSql("d", "2026-01-03 00:00:00.0000000"));
+            await ExecAsync(conn, InsertTaskSql("a", "2026-01-01 00:00:00.0000000"));
+            await ExecAsync(conn, InsertTaskSql("c", "2026-01-03 00:00:00.0000000"));
+            await ExecAsync(conn, InsertTaskSql("b", "2026-01-02 00:00:00.0000000"));
+
+            migrator.Migrate();
+        }
+
+        await using var verify = new ClaudeDoDbContext(_options);
+        var byId = await verify.Tasks.AsNoTracking().ToDictionaryAsync(t => t.Id, t => t.Number);
+
+        Assert.Equal(1, byId["a"]);
+        Assert.Equal(2, byId["b"]);
+        Assert.Equal(3, byId["c"]);
+        Assert.Equal(4, byId["d"]);
+
+        var numbers = byId.Values.OrderBy(n => n).ToList();
+        Assert.Equal(new[] { 1, 2, 3, 4 }, numbers);
+
+        var settings = await verify.AppSettings.AsNoTracking().FirstAsync(s => s.Id == AppSettingsEntity.SingletonId);
+        Assert.Equal(5, settings.NextTaskNumber);
+    }
+
+    private static string InsertTaskSql(string id, string createdAt) => $"""
+        INSERT INTO tasks (id, list_id, title, status, created_at)
+        VALUES ('{id}', 'l1', 'Task {id}', 'idle', '{createdAt}');
+        """;
+
+    private static async Task ExecAsync(System.Data.Common.DbConnection conn, string sql)
+    {
+        await using var cmd = conn.CreateCommand();
+        cmd.CommandText = sql;
+        await cmd.ExecuteNonQueryAsync();
+    }
+}