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/TreeMergeTests.cs

@@ -74,9 +74,9 @@ public sealed class TreeMergeTests : IDisposable
         using (var ctx = db.CreateContext())
         {
             ctx.Lists.Add(new ListEntity { Id = listId, Name = "L", WorkingDir = repo.RepoDir, CreatedAt = DateTime.UtcNow });
-            ctx.Tasks.Add(new TaskEntity { Id = parentId, ListId = listId, Title = "Parent",
+            ctx.Tasks.Add(new TaskEntity { Id = parentId, ListId = listId, Title = "Parent", Number = 1,
                 Status = TaskStatus.WaitingForReview, PlanningPhase = PlanningPhase.None, SortOrder = 0, CreatedAt = DateTime.UtcNow });
-            ctx.Tasks.Add(new TaskEntity { Id = childId, ListId = listId, Title = "Child",
+            ctx.Tasks.Add(new TaskEntity { Id = childId, ListId = listId, Title = "Child", Number = 2,
                 Status = TaskStatus.Done, ParentTaskId = parentId, SortOrder = 1, CreatedAt = DateTime.UtcNow });
             await ctx.SaveChangesAsync();