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.

src/ClaudeDo.Data/Models/TaskEntity.cs

@@ -22,6 +22,10 @@ public enum PlanningPhase
 public sealed class TaskEntity
 {
     public required string Id { get; init; }
+    // Global, monotonically increasing, never-reused display/lookup handle shown as `#123`.
+    // Allocated by TaskNumberAllocator from AppSettingsEntity.NextTaskNumber -- never MAX(number)+1,
+    // which would reissue a deleted task's number. The GUID Id stays the real identity everywhere else.
+    public int Number { get; set; }
     public required string ListId { get; init; }
     public required string Title { get; set; }
     public string? Description { get; set; }