fix(storage): address review on PR #799 (stale comments, docs deprecation, unit test)

claude-review on #799 flagged:

1. Stale inline comment in ``initialize()`` (line 466) still said
   "Postgres uses a small bounded pool". Updated to reflect both
   backends now use NullPool.

2. Stale ``close()`` docstring referenced pool-size starving
   max_connections — irrelevant with NullPool. Replaced with the
   NullPool-aware rationale (dispose still tears down in-flight
   asyncpg connections cleanly).

3. ``docs/configuration.md`` actively directed operators to tune
   DATABASE_POOL_SIZE / DATABASE_MAX_OVERFLOW, with worked
   examples and pool math. Both are now deprecated no-ops; the
   table entries explain the deprecation and link to PR #799.
   Operators reading the docs will no longer be confused into
   tuning settings that don't do anything.

4. ``config.py`` comment for the deprecated fields updated to
   record the deprecation. Validators are intentionally kept
   (still reject < 1 / < 0) so misconfigured deploys fail loudly
   rather than silently — the reviewer flagged this as a minor
   UX wart but explicitly "not a blocker"; the docs change in (3)
   keeps operators away from the config altogether.

5. New ``tests/unit/test_storage_engine.py`` with three tests:
   - ``test_postgres_engine_uses_nullpool`` — pins ``isinstance(
     engine.pool, NullPool)`` so a refactor back to QueuePool /
     SingletonThreadPool can't silently re-introduce the cross-
     event-loop crashes.
   - ``test_postgres_engine_ignores_pool_sizing_settings`` —
     setting DATABASE_POOL_SIZE / DATABASE_MAX_OVERFLOW to huge
     values must not change pool type (proves the deprecated
     fields are wired-up no-ops).
   - ``test_postgres_engine_missing_asyncpg_driver_message`` —
     guards the existing actionable-error branch when the
     optional ``[postgres]`` extra isn't installed.

Verified:
- ``uv run pytest tests/unit/`` — 1028 passed
- ``uv run ruff check`` clean on the touched python files

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

nextcloud_mcp_server/auth/storage.py

@@ -463,10 +463,11 @@ class RefreshTokenStorage:
             if Path(self.db_path).exists():
                 os.chmod(self.db_path, 0o600)
 
-        # Create the shared async engine for the chosen backend. SQLite uses
-        # NullPool (per-call connections, matches the prior aiosqlite-direct
-        # behavior); Postgres uses a small bounded pool — see
-        # ``_build_postgres_engine`` for sizing rationale.
+        # Create the shared async engine for the chosen backend. Both
+        # SQLite and Postgres use NullPool (per-call connections, no
+        # cross-loop bookkeeping). SQLite mirrors the prior
+        # aiosqlite-direct behavior; see ``_build_postgres_engine`` for
+        # the Postgres rationale.
         if is_sqlite:
             self.engine = create_async_engine(
                 self.database_url,
@@ -598,12 +599,11 @@ class RefreshTokenStorage:
     async def close(self) -> None:
         """Dispose the underlying AsyncEngine on shutdown.
 
-        Without an explicit dispose, asyncpg's pooled connections leak
-        server-side slots until the Postgres
-        ``idle_in_transaction_session_timeout`` reaps them — with the
-        small pool defaults and frequent k8s rolling restarts this can
-        starve ``max_connections``. Idempotent: safe to call from any
-        number of shutdown hooks.
+        With ``NullPool`` the dispose call has no idle pool to drain,
+        but it still cleanly tears down any in-flight asyncpg
+        connections held by active checkouts so shutdown hooks don't
+        leave dangling transports behind. Idempotent: safe to call
+        from any number of shutdown hooks.
         """
         if self.engine is None:
             return