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/Migrations/ClaudeDoDbContextModelSnapshot.cs

@@ -76,6 +76,12 @@ namespace ClaudeDo.Data.Migrations
                         .HasColumnType("TEXT")
                         .HasColumnName("model_presets");
 
+                    b.Property<int>("NextTaskNumber")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("INTEGER")
+                        .HasDefaultValue(1)
+                        .HasColumnName("next_task_number");
+
                     b.Property<string>("RepoImportFolders")
                         .HasColumnType("TEXT")
                         .HasColumnName("repo_import_folders");
@@ -164,6 +170,7 @@ namespace ClaudeDo.Data.Migrations
                             DefaultPermissionMode = "auto",
                             MaxParallelExecutions = 1,
                             MaxTurnsCeiling = 80,
+                            NextTaskNumber = 1,
                             StandupWeekday = 3,
                             UsageGateFiveHourPct = 80,
                             UsageGateSevenDayPct = 90,
@@ -548,6 +555,10 @@ namespace ClaudeDo.Data.Migrations
                         .HasColumnType("TEXT")
                         .HasColumnName("notes");
 
+                    b.Property<int>("Number")
+                        .HasColumnType("INTEGER")
+                        .HasColumnName("number");
+
                     b.Property<string>("ParentTaskId")
                         .HasColumnType("TEXT")
                         .HasColumnName("parent_task_id");
@@ -632,6 +643,10 @@ namespace ClaudeDo.Data.Migrations
                     b.HasIndex("ListId")
                         .HasDatabaseName("idx_tasks_list_id");
 
+                    b.HasIndex("Number")
+                        .IsUnique()
+                        .HasDatabaseName("idx_tasks_number");
+
                     b.HasIndex("ParentTaskId")
                         .HasDatabaseName("idx_tasks_parent_task_id");