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/TaskNumberAllocator.cs

@@ -0,0 +1,46 @@
+using ClaudeDo.Data.Models;
+using Microsoft.EntityFrameworkCore;
+
+namespace ClaudeDo.Data;
+
+// Allocates TaskEntity.Number and inserts the task. Never MAX(number)+1 -- that would reissue a
+// deleted task's number. Allocation is a single atomic UPDATE...RETURNING against the
+// app_settings singleton row, which SQLite's single-writer model already serializes.
+//
+// The allocation and the insert are deliberately two separate statements, each committed on its
+// own, rather than one shared transaction: a shared transaction would roll the counter bump back
+// together with a failed insert, so a retry would hand out the exact same (still colliding)
+// number every time and the bounded retry below would be pointless. Committing the bump
+// unconditionally means a retry after a collision always advances to a fresh number, at the cost
+// of burning the collided number as an unused gap -- which is fine, gaps are expected.
+public static class TaskNumberAllocator
+{
+    private const int MaxAttempts = 5;
+
+    public static async Task AddWithNumberAsync(
+        ClaudeDoDbContext context, TaskEntity entity, CancellationToken ct = default)
+    {
+        for (var attempt = 1; ; attempt++)
+        {
+            var settings = await context.AppSettings.FromSqlRaw(
+                """
+                UPDATE app_settings SET next_task_number = next_task_number + 1
+                WHERE id = {0}
+                RETURNING *
+                """, AppSettingsEntity.SingletonId).AsNoTracking().ToListAsync(ct);
+
+            entity.Number = settings[0].NextTaskNumber - 1;
+            context.Tasks.Add(entity);
+
+            try
+            {
+                await context.SaveChangesAsync(ct);
+                return;
+            }
+            catch (DbUpdateException) when (attempt < MaxAttempts)
+            {
+                context.Entry(entity).State = EntityState.Detached;
+            }
+        }
+    }
+}