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.Worker.Tests/State/TaskStateServiceTests.cs

@@ -25,6 +25,7 @@ public sealed class TaskStateServiceTests : IDisposable
     private readonly ITaskStateService _sut;
     private readonly string _listId;
     private readonly List<GitRepoFixture> _repos = new();
+    private int _numberSeed;
 
     private static bool GitAvailable => GitRepoFixture.IsGitAvailable();
 
@@ -75,6 +76,7 @@ public sealed class TaskStateServiceTests : IDisposable
             Id = id,
             ListId = _listId,
             Title = "task",
+            Number = ++_numberSeed,
             Status = status,
             PlanningPhase = phase,
             CreatedAt = DateTime.UtcNow,