feat: account deletion, username reuse, and real-time username availability

- Add account deletion with tombstone (isDeleted flag), password/username
  confirmation, owned-space guard, and full cleanup transaction
- Free deleted usernames by renaming to !deleted:<id> so they can be reused
- Add migration to retroactively free usernames from already-tombstoned users
- Add GET /api/auth/check-username endpoint with rate limiting for real-time
  availability checking during registration
- Add debounced username availability indicator on registration Step 1
- Add DeleteAccountModal with federation-aware remote account cleanup
- Add federation ops utility for remote instance management
- Update sanitizeUser to anonymize deleted user profiles
- Add instance store improvements and connected instances modal updates

packages/server/src/db/schema.ts

@@ -16,6 +16,7 @@ export const users = sqliteTable('users', {
   accentColor: text('accent_color'),
   avatarColor: text('avatar_color'),
   bio: text('bio'),
+  isDeleted: integer('is_deleted').default(0),
   createdAt: integer('created_at').notNull(),
 });