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/External/ExternalMcpServiceTests.cs

@@ -61,6 +61,7 @@ public sealed class ExternalMcpServiceTests : IDisposable
     private readonly HubBroadcaster _broadcaster;
     private readonly List<GitRepoFixture> _repos = new();
     private readonly List<(string repoDir, string wtPath)> _worktreeCleanups = new();
+    private int _numberSeed;
 
     private static bool GitAvailable => GitRepoFixture.IsGitAvailable();
 
@@ -120,6 +121,7 @@ public sealed class ExternalMcpServiceTests : IDisposable
             ParentTaskId = parentId,
             CreatedAt = DateTime.UtcNow,
             CommitType = "chore",
+            Number = ++_numberSeed,
         };
         await _tasks.AddAsync(task);
         return task;
@@ -532,9 +534,9 @@ public sealed class ExternalMcpServiceTests : IDisposable
         {
             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 = "plan", CreatedAt = DateTime.UtcNow,
-                Status = TaskStatus.WaitingForReview, PlanningPhase = PlanningPhase.Finalized });
+                Status = TaskStatus.WaitingForReview, PlanningPhase = PlanningPhase.Finalized, Number = ++_numberSeed });
             ctx.Tasks.Add(new TaskEntity { Id = childId, ListId = listId, Title = "child", CreatedAt = DateTime.UtcNow,
-                ParentTaskId = parentId, Status = TaskStatus.Done, SortOrder = 1 });
+                ParentTaskId = parentId, Status = TaskStatus.Done, SortOrder = 1, Number = ++_numberSeed });
 
             var wtPath = Path.Combine(Path.GetTempPath(), $"wt_{Guid.NewGuid():N}");
             _worktreeCleanups.Add((repo.RepoDir, wtPath));
@@ -582,11 +584,11 @@ public sealed class ExternalMcpServiceTests : IDisposable
         {
             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 = "plan", CreatedAt = DateTime.UtcNow,
-                Status = TaskStatus.WaitingForReview, PlanningPhase = PlanningPhase.Finalized });
+                Status = TaskStatus.WaitingForReview, PlanningPhase = PlanningPhase.Finalized, Number = ++_numberSeed });
             ctx.Tasks.Add(new TaskEntity { Id = normalChildId, ListId = listId, Title = "normal child", CreatedAt = DateTime.UtcNow,
-                ParentTaskId = parentId, Status = TaskStatus.Done, SortOrder = 1 });
+                ParentTaskId = parentId, Status = TaskStatus.Done, SortOrder = 1, Number = ++_numberSeed });
             ctx.Tasks.Add(new TaskEntity { Id = blockedChildId, ListId = listId, Title = "blocked child", CreatedAt = DateTime.UtcNow,
-                ParentTaskId = parentId, Status = TaskStatus.Done, SortOrder = 2, RoadblockCount = 1 });
+                ParentTaskId = parentId, Status = TaskStatus.Done, SortOrder = 2, RoadblockCount = 1, Number = ++_numberSeed });
 
             var normalWtPath = Path.Combine(Path.GetTempPath(), $"wt_{Guid.NewGuid():N}");
             _worktreeCleanups.Add((repo.RepoDir, normalWtPath));
@@ -773,6 +775,7 @@ public sealed class ExternalMcpServiceTests : IDisposable
             Status = TaskStatus.Idle,
             CreatedAt = DateTime.UtcNow,
             CommitType = "chore",
+            Number = ++_numberSeed,
         };
         await _tasks.AddAsync(idleUnblocked);
 
@@ -786,6 +789,7 @@ public sealed class ExternalMcpServiceTests : IDisposable
             BlockedByTaskId = "idle-unblocked",
             CreatedAt = DateTime.UtcNow,
             CommitType = "chore",
+            Number = ++_numberSeed,
         };
         await _tasks.AddAsync(idleBlocked);
 
@@ -798,6 +802,7 @@ public sealed class ExternalMcpServiceTests : IDisposable
             Status = TaskStatus.Done,
             CreatedAt = DateTime.UtcNow,
             CommitType = "chore",
+            Number = ++_numberSeed,
         };
         await _tasks.AddAsync(doneTask);
 
@@ -811,6 +816,7 @@ public sealed class ExternalMcpServiceTests : IDisposable
             IsMyDay = true,
             CreatedAt = DateTime.UtcNow,
             CommitType = "chore",
+            Number = ++_numberSeed,
         };
         await _tasks.AddAsync(myDayTask);
 
@@ -826,6 +832,7 @@ public sealed class ExternalMcpServiceTests : IDisposable
             Status = TaskStatus.Idle,
             CreatedAt = DateTime.UtcNow,
             CommitType = "chore",
+            Number = ++_numberSeed,
         };
         await _tasks.AddAsync(excludedRepoTask);
 
@@ -841,6 +848,7 @@ public sealed class ExternalMcpServiceTests : IDisposable
             Status = TaskStatus.Idle,
             CreatedAt = DateTime.UtcNow,
             CommitType = "chore",
+            Number = ++_numberSeed,
         };
         await _tasks.AddAsync(noRepoTask);
 
@@ -1792,9 +1800,9 @@ public sealed class ExternalMcpServiceTests : IDisposable
         {
             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 = "plan", CreatedAt = DateTime.UtcNow,
-                Status = TaskStatus.WaitingForReview, PlanningPhase = PlanningPhase.Finalized });
+                Status = TaskStatus.WaitingForReview, PlanningPhase = PlanningPhase.Finalized, Number = ++_numberSeed });
             ctx.Tasks.Add(new TaskEntity { Id = childId, ListId = listId, Title = "child", CreatedAt = DateTime.UtcNow,
-                ParentTaskId = parentId, Status = TaskStatus.Done, SortOrder = 1 });
+                ParentTaskId = parentId, Status = TaskStatus.Done, SortOrder = 1, Number = ++_numberSeed });
 
             var wtPath = Path.Combine(Path.GetTempPath(), $"wt_{Guid.NewGuid():N}");
             _worktreeCleanups.Add((repo.RepoDir, wtPath));