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/AppSettingsEntity.cs

@@ -6,6 +6,11 @@ public sealed class AppSettingsEntity
 
     public int Id { get; set; } = SingletonId;
 
+    // Persistent counter backing TaskNumberAllocator: the next task number to hand out. Lives on
+    // the singleton row so allocation is a single atomic UPDATE...RETURNING in the task's own
+    // insert transaction, never a MAX(number)+1 recompute.
+    public int NextTaskNumber { get; set; } = 1;
+
     public string DefaultClaudeInstructions { get; set; } = string.Empty;
     public string DefaultModel { get; set; } = "sonnet";
     public int DefaultMaxTurns { get; set; } = 40;