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/External/QueueStateMcpToolsTests.cs

@@ -22,6 +22,7 @@ public sealed class QueueStateMcpToolsTests : IDisposable
     private readonly ListRepository _listRepo;
     private readonly WorkerConfig _cfg;
     private readonly string _tempDir;
+    private int _numberSeed;
 
     public QueueStateMcpToolsTests()
     {
@@ -86,6 +87,7 @@ public sealed class QueueStateMcpToolsTests : IDisposable
             IsManual = isManual,
             BlockedByTaskId = blockedByTaskId,
             ScheduledFor = scheduledFor,
+            Number = ++_numberSeed,
         };
         // Bypass TaskRepository.AddAsync, which overwrites SortOrder with max(listId)+1 --
         // these tests need to control SortOrder directly to exercise queue pick order.