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/config.py

@@ -520,12 +520,14 @@ class Settings:
     # or supply a private-CA bundle.
     database_verify_ssl: bool | None = None
     database_ca_bundle: str | None = None
-    # Postgres connection pool sizing (ADR-026). The asyncpg engine maps
-    # these to its underlying QueuePool. Per-pod max = pool_size +
-    # max_overflow. Defaults are intentionally small (2 + 5 = 7) because
-    # asyncpg connections are single-flight and the typical MCP workload
-    # is light read-mostly point lookups. Validate >= 1 / >= 0 in
-    # __post_init__.
+    # Postgres connection pool sizing — DEPRECATED, retained for
+    # backward compatibility. The asyncpg engine switched to NullPool
+    # in #799 (cross-event-loop crashes under anyio TaskGroups made
+    # the original QueuePool + pool_pre_ping setup unsafe). These
+    # fields no longer affect the Postgres engine; the validators
+    # below still reject invalid values so misconfigured deploys
+    # fail loudly rather than silently. See ADR-026 § Connection
+    # pool and docs/configuration.md.
     database_pool_size: int = 2
     database_max_overflow: int = 5