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/UiVm/TasksIslandViewModelPlanningTests.cs

@@ -348,6 +348,7 @@ public class TasksIslandViewModelPlanningTests
 public sealed class TasksIslandViewModelMyDayTests : IDisposable
 {
     private readonly DbFixture _db = new();
+    private int _numberSeed;
     public void Dispose() => _db.Dispose();
 
     [Fact]
@@ -361,6 +362,7 @@ public sealed class TasksIslandViewModelMyDayTests : IDisposable
             ctx.Tasks.Add(new TaskEntity
             {
                 Id = taskId, ListId = listId, Title = "T", CreatedAt = DateTime.UtcNow,
+                Number = ++_numberSeed,
                 Status = TaskStatus.Idle, IsMyDay = false,
             });
             await ctx.SaveChangesAsync();
@@ -391,16 +393,19 @@ public sealed class TasksIslandViewModelMyDayTests : IDisposable
             ctx.Tasks.Add(new TaskEntity
             {
                 Id = parentId, ListId = listId, Title = "P", CreatedAt = DateTime.UtcNow,
+                Number = ++_numberSeed,
                 Status = TaskStatus.WaitingForChildren, PlanningPhase = PlanningPhase.Finalized, IsMyDay = true,
             });
             ctx.Tasks.Add(new TaskEntity
             {
                 Id = child1, ListId = listId, Title = "C1", CreatedAt = DateTime.UtcNow,
+                Number = ++_numberSeed,
                 Status = TaskStatus.Idle, ParentTaskId = parentId, IsMyDay = true,
             });
             ctx.Tasks.Add(new TaskEntity
             {
                 Id = child2, ListId = listId, Title = "C2", CreatedAt = DateTime.UtcNow,
+                Number = ++_numberSeed,
                 Status = TaskStatus.Idle, ParentTaskId = parentId, IsMyDay = true,
             });
             await ctx.SaveChangesAsync();