fix(storage): address PR #798 round-3 review (SonarQube + pool sizing + RETURNING test)

Round-3 fixes. Two threads:
- 9 OPEN SonarQube issues caused the "E Security Rating on New Code"
  gate failure. The bot's diagnosis (sa.text(text_sql) → SQL injection)
  was a wrong guess; the actual SQ rules firing were different.
- Bot's substantive concerns: pool defaults too aggressive,
  delete_browser_session RETURNING path untested on Postgres,
  schema_version legacy table created on Postgres, stale module
  docstring.
- User's underlying question on the pool: "isn't 1 connection enough?"
  Right-sized to 2+5 and documented the concurrency model in ADR-026
  so the rationale is durable.

SonarQube quality-gate fixes (clears all 9 OPEN issues)
-------------------------------------------------------
- BLOCKER S6418: rename `SECRET` constant in test_storage_logging.py
  to `SENTINEL_PASSWORD_FRAGMENT` + NOSONAR with rationale.
- CRITICAL S3776: extract `_build_postgres_engine()` from
  `initialize()` (was complexity 26 > 15); incidentally creates a
  clean unit-test seam for engine args.
- CRITICAL S4423: `ssl.create_default_context(cafile=...)` is flagged
  as "weak protocol" — Python 3.10+ already negotiates the strongest
  available protocol. Explicitly pass `purpose=ssl.Purpose.SERVER_AUTH`
  and NOSONAR with the Python-version rationale.
- MAJOR S3358: split the TLS-mode nested ternary in the engine
  factory into a `_describe_ssl_arg()` helper.
- MAJOR S2068 ×3: bind test app-password literals to local vars and
  put `# NOSONAR S2068` on the same line as the literal (anchoring
  requirement) instead of on the closing paren.
- MINOR S7503 ×2: `# NOSONAR S7503` on `_Cursor.__aenter__/__aexit__`
  — they MUST be `async` per the context-manager protocol.

Pool sizing right-sized (answers "why so many connections?")
------------------------------------------------------------
- `DATABASE_POOL_SIZE` default 10 → **2**.
- `DATABASE_MAX_OVERFLOW` default 20 → **5**.
- Per-pod max drops from 30 to 7. With 3 replicas, total = 21
  connections (was 90) — well under managed-Postgres
  `max_connections=100`.
- New INFO log at startup: `Postgres engine ready: pool_size=N
  max_overflow=M (per-pod max K connections)`. Surfaces the active
  sizing without grepping config.
- New ADR-026 § "Concurrency model and pool sizing" explains
  asyncpg's single-flight connection semantics, the MCP workload
  shape (read-mostly point lookups), why-not-1 (multi-user
  serialization), and the tune-up/tune-down recipe.
- `docs/configuration.md` table updated with new defaults +
  homelab-vs-prod tuning guidance, linking the ADR.

RETURNING path covered on Postgres
----------------------------------
- New `test_browser_session_delete_returning` exercises the
  `DELETE … RETURNING user_id` path — the only RETURNING clause in
  the storage layer and the most dialect-sensitive SQL in this PR.
  Asserts both present-row (returns True, row gone) and absent-row
  (returns False) branches.

Schema portability polish
-------------------------
- `alembic 001`: gate `schema_version` table creation on
  `op.get_bind().dialect.name == "sqlite"`. The table exists purely
  to match the fingerprint of pre-Alembic SQLite databases; fresh
  Postgres installs no longer carry the dead legacy table.

Misc polish
-----------
- Module docstring: "SQLite-based" → "SQL-backed", with a sentence
  on the DATABASE_URL opt-in and an ADR-026 link.
- Comment on `_wrap_row` noting `row._mapping` is the documented
  RowMapping accessor in SQLAlchemy 2.x despite the underscore.

Skipped (rationale in PR reply)
-------------------------------
- `_qmark_to_named` SQL-comment handling: docstring already notes
  the limitation; no `?` in storage SQL comments today.
- Module-level `anyio.Lock()`: established precedent confirmed by
  the bot itself.
- `get_audit_logs` `SELECT *`: pre-existing pattern, out of scope.

Verification
------------
- `uv run pytest tests/unit/` — 1025 passed.
- `TEST_DATABASE_URL=… uv run pytest tests/integration/test_storage_postgres.py -m postgres` — 7 passed.
- `ruff check && ruff format --check && ty check` — clean.
- Confirmed `schema_version` absent on fresh Postgres, still present
  on fresh SQLite.

Tracked on Astrolabe Cloud POC board, card #99.

---

_This PR was generated with the help of AI, and reviewed by a Human_

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

tests/unit/test_storage_logging.py

@@ -19,14 +19,19 @@ from nextcloud_mcp_server.config import mask_db_password
 pytestmark = pytest.mark.unit
 
 
-SECRET = "uniqueSecretSentinel123"
+# Synthetic leak-detection sentinel — embedded into test-only URLs so we
+# can grep ``caplog`` and prove the masking path never emits the literal
+# password substring. Not a real credential. NOSONAR S6418
+SENTINEL_PASSWORD_FRAGMENT = "uniqueSecretSentinel123"  # NOSONAR S6418
 
 
 def test_mask_db_password_postgres():
     """Postgres URL passwords are replaced with the SQLAlchemy ``***`` token."""
-    url = f"postgresql+asyncpg://mcp:{SECRET}@db.example.com:5432/mcp"
+    url = (
+        f"postgresql+asyncpg://mcp:{SENTINEL_PASSWORD_FRAGMENT}@db.example.com:5432/mcp"
+    )
     masked = mask_db_password(url)
-    assert SECRET not in masked
+    assert SENTINEL_PASSWORD_FRAGMENT not in masked
     assert "mcp" in masked  # username preserved
     assert "db.example.com" in masked  # host preserved
 
@@ -45,9 +50,9 @@ def test_mask_db_password_handles_unparseable_url():
     less-pretty masked value — never let credentials leak just because the
     URL shape was unexpected.
     """
-    url = f"weird-scheme://user:{SECRET}@host/db?ssl=disable"
+    url = f"weird-scheme://user:{SENTINEL_PASSWORD_FRAGMENT}@host/db?ssl=disable"
     masked = mask_db_password(url)
-    assert SECRET not in masked
+    assert SENTINEL_PASSWORD_FRAGMENT not in masked
 
 
 async def test_storage_init_does_not_log_password(caplog):
@@ -75,6 +80,6 @@ async def test_storage_init_does_not_log_password(caplog):
     # but if a future change reformatted DATABASE_URL into the message it
     # would). Stay paranoid.
     for rec in caplog.records:
-        assert SECRET not in rec.getMessage(), (
+        assert SENTINEL_PASSWORD_FRAGMENT not in rec.getMessage(), (
             f"Credential sentinel leaked into log: {rec.getMessage()!r}"
         )