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/Usage/RunningTaskProbeTests.cs

@@ -8,6 +8,7 @@ namespace ClaudeDo.Worker.Tests.Usage;
 public sealed class RunningTaskProbeTests : IDisposable
 {
     private readonly DbFixture _db = new();
+    private int _numberSeed;
 
     public void Dispose() => _db.Dispose();
 
@@ -21,6 +22,7 @@ public sealed class RunningTaskProbeTests : IDisposable
             Id = Guid.NewGuid().ToString(),
             ListId = list.Id,
             Title = $"task-{status}",
+            Number = ++_numberSeed,
             Status = status,
             CreatedAt = DateTime.UtcNow,
         });