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/SuggestImprovementTests.cs

@@ -12,6 +12,7 @@ namespace ClaudeDo.Worker.Tests;
 public sealed class SuggestImprovementTests : IDisposable
 {
     private readonly DbFixture _db = new();
+    private int _numberSeed;
     public void Dispose() => _db.Dispose();
 
     private static TaskRunMcpContextAccessor AccessorFor(string callerTaskId)
@@ -26,7 +27,7 @@ public sealed class SuggestImprovementTests : IDisposable
         using var ctx = _db.CreateContext();
         if (!ctx.Lists.Any())
             ctx.Lists.Add(new ListEntity { Id = "l1", Name = "L", CreatedAt = DateTime.UtcNow });
-        ctx.Tasks.Add(new TaskEntity { Id = id, ListId = "l1", Title = "Caller",
+        ctx.Tasks.Add(new TaskEntity { Id = id, ListId = "l1", Title = "Caller", Number = ++_numberSeed,
             Status = TaskStatus.Running, ParentTaskId = parentId, CommitType = "feat", CreatedAt = DateTime.UtcNow });
         await ctx.SaveChangesAsync();
     }