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/Planning/PlanningAggregatorTests.cs

@@ -67,7 +67,7 @@ public class PlanningAggregatorTests : IDisposable
         ctx.Tasks.Add(new TaskEntity
         {
             Id = parentId, ListId = listId, Title = "plan", CreatedAt = DateTime.UtcNow,
-            Status = TaskStatus.Idle, PlanningPhase = PlanningPhase.Active, SortOrder = 0,
+            Status = TaskStatus.Idle, PlanningPhase = PlanningPhase.Active, SortOrder = 0, Number = 1,
         });
 
         // Two children (sorted A then B).
@@ -76,12 +76,12 @@ public class PlanningAggregatorTests : IDisposable
         ctx.Tasks.Add(new TaskEntity
         {
             Id = subA, ListId = listId, Title = "child A", CreatedAt = DateTime.UtcNow,
-            ParentTaskId = parentId, Status = TaskStatus.Done, SortOrder = 1,
+            ParentTaskId = parentId, Status = TaskStatus.Done, SortOrder = 1, Number = 2,
         });
         ctx.Tasks.Add(new TaskEntity
         {
             Id = subB, ListId = listId, Title = "child B", CreatedAt = DateTime.UtcNow,
-            ParentTaskId = parentId, Status = TaskStatus.Done, SortOrder = 2,
+            ParentTaskId = parentId, Status = TaskStatus.Done, SortOrder = 2, Number = 3,
         });
         await ctx.SaveChangesAsync();
 
@@ -171,19 +171,19 @@ public class PlanningAggregatorTests : IDisposable
         ctx.Tasks.Add(new TaskEntity
         {
             Id = parentId, ListId = listId, Title = "plan", CreatedAt = DateTime.UtcNow,
-            Status = TaskStatus.Idle, PlanningPhase = PlanningPhase.Active, SortOrder = 0,
+            Status = TaskStatus.Idle, PlanningPhase = PlanningPhase.Active, SortOrder = 0, Number = 1,
         });
         var subA = Guid.NewGuid().ToString();
         var subB = Guid.NewGuid().ToString();
         ctx.Tasks.Add(new TaskEntity
         {
             Id = subA, ListId = listId, Title = "A", CreatedAt = DateTime.UtcNow,
-            ParentTaskId = parentId, Status = TaskStatus.Done, SortOrder = 1,
+            ParentTaskId = parentId, Status = TaskStatus.Done, SortOrder = 1, Number = 2,
         });
         ctx.Tasks.Add(new TaskEntity
         {
             Id = subB, ListId = listId, Title = "B", CreatedAt = DateTime.UtcNow,
-            ParentTaskId = parentId, Status = TaskStatus.Done, SortOrder = 2,
+            ParentTaskId = parentId, Status = TaskStatus.Done, SortOrder = 2, Number = 3,
         });
         await ctx.SaveChangesAsync();