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/Configuration/AppSettingsEntityConfiguration.cs

@@ -13,6 +13,9 @@ public class AppSettingsEntityConfiguration : IEntityTypeConfiguration<AppSettin
         builder.HasKey(s => s.Id);
         builder.Property(s => s.Id).HasColumnName("id").ValueGeneratedNever();
 
+        builder.Property(s => s.NextTaskNumber)
+            .HasColumnName("next_task_number").IsRequired().HasDefaultValue(1);
+
         builder.Property(s => s.DefaultClaudeInstructions)
             .HasColumnName("default_claude_instructions").IsRequired().HasDefaultValue(string.Empty);
         builder.Property(s => s.DefaultModel)