phase: 110_fix_sse_db_pool_exhaustion

---

**Phase 110 — Fix SSE DB Connection Pool Exhaustion (SEC-14-04): COMPLETE**

**What was implemented/verified:**
- All three tasks (pool config, short-lived sessions, concurrency cap) were already implemented in code
- Fixed `tests/integration/test_chat_db_sessions.py` — added FakeChatLLM mock, fixed LLM signature (`tools=` not `_tools=`), used `fastapi_app.dependency_overrides` instead of `client.app.dependency_overrides`
- Fixed `tests/e2e/test_chat_db_pool.py` — added FakeChatLLM mock, fixed admin password to match `tests/conftest.py`, removed unused imports
- Fixed lint errors (unused imports, import order) in both test files

**Test / lint / coverage results:**
- `uv run pytest` → 2350 passed, 1 warning, 56.4s
- `uv run pytest --cov=app --cov-report=term-missing` → 99% coverage (4065 lines, 16 uncovered)
- `uv run pytest tests/e2e/test_chat_db_pool.py -v --no-cov` → 3 passed
- `uv run pytest tests/integration/test_chat_db_sessions.py -v --no-cov` → 4 passed
- `uv run pytest tests/integration/test_chat_concurrency.py -v --no-cov` → 11 passed
- `uv run pytest tests/unit/test_db_pool_config.py -v --no-cov` → 14 passed
- `uv run pytest tests/unit/test_agent_short_lived_sessions.py -v --no-cov` → 7 passed
- `uv run ruff check .` → all checks passed
- `uv run pyright` → 0 errors, 0 warnings

**Completion criteria:**
- [✓] `app/db.py::create_engine` receives explicit `pool_size=5`, `max_overflow=10`, `pool_recycle=3600` from settings
- [✓] `run_agent` accepts `db_factory: Callable[[], Session]` and creates short-lived sessions per tool call
- [✓] Each tool round uses a separate DB session closed after the tool result
- [✓] Concurrency cap (`BOR_CHAT_MAX_CONCURRENT`, default 10) limits concurrent turns; excess get 503
- [✓] All test gates green, coverage 99%, lint/types clean

**Notable decisions:** Tests needed LLM mocking (the original test files lacked `FakeChatLLM` mocks, causing hangs on real LLM calls).

**Next pending phase:** None — this is the last phase in `todo/`.

tests/unit/test_db_pool_config.py

@@ -0,0 +1,108 @@
+"""Unit tests for DB pool configuration (SEC-14-04, phase 106, task 01).
+
+Verifies that:
+- Settings expose db_pool_size, db_pool_max_overflow, db_pool_recycle
+  with correct defaults and validators.
+- create_engine() receives the pool kwargs from settings.
+"""
+from __future__ import annotations
+
+import contextlib
+
+import pytest
+
+from app.config import Settings
+
+
+class TestSettingsDefaults:
+    """Pool config defaults match SQLAlchemy implicit defaults."""
+
+    def test_pool_size_default(self):
+        assert Settings().db_pool_size == 5
+
+    def test_pool_max_overflow_default(self):
+        assert Settings().db_pool_max_overflow == 10
+
+    def test_pool_recycle_default(self):
+        assert Settings().db_pool_recycle == 3600
+
+
+class TestSettingsCustomValues:
+    """Custom values round-trip correctly."""
+
+    def test_custom_all_three(self):
+        s = Settings(
+            db_pool_size=10,
+            db_pool_max_overflow=20,
+            db_pool_recycle=1800,
+        )
+        assert s.db_pool_size == 10
+        assert s.db_pool_max_overflow == 20
+        assert s.db_pool_recycle == 1800
+
+    def test_custom_pool_size_only(self):
+        s = Settings(db_pool_size=8)
+        assert s.db_pool_size == 8
+        assert s.db_pool_max_overflow == 10
+        assert s.db_pool_recycle == 3600
+
+
+class TestValidators:
+    """Pool config validators reject invalid values."""
+
+    def test_pool_size_zero_raises(self):
+        with pytest.raises(ValueError, match="db_pool_size must be >= 1"):
+            Settings(db_pool_size=0)
+
+    def test_pool_size_negative_raises(self):
+        with pytest.raises(ValueError, match="db_pool_size must be >= 1"):
+            Settings(db_pool_size=-5)
+
+    def test_pool_max_overflow_negative_raises(self):
+        with pytest.raises(ValueError, match="db_pool_max_overflow must be >= 0"):
+            Settings(db_pool_max_overflow=-1)
+
+    def test_pool_max_overflow_zero_is_legal(self):
+        s = Settings(db_pool_max_overflow=0)
+        assert s.db_pool_max_overflow == 0
+
+    def test_pool_recycle_zero_is_legal(self):
+        """pool_recycle=0 means never recycle — legal, just aggressive."""
+        s = Settings(db_pool_recycle=0)
+        assert s.db_pool_recycle == 0
+
+
+class TestEngineKwargs:
+    """create_engine() receives the correct pool parameters from settings."""
+
+    def test_create_engine_pool_pre_ping_true(self):
+        """pool_pre_ping must remain True (connection health check)."""
+        from app import db  # noqa: F811
+
+        # The engine's pool options include pool_pre_ping=True.
+        # We verify by checking the pool's _pre_ping attribute.
+        assert db.engine.pool._pre_ping is True
+
+    def test_create_engine_pool_recycle(self):
+        """pool_recycle defaults to 3600 seconds."""
+        from app import db  # noqa: F811
+
+        assert db.engine.pool._recycle == 3600
+
+    def test_sessionlocal_still_callable(self):
+        """SessionLocal remains a valid session factory."""
+        from app import db  # noqa: F811
+
+        assert callable(db.SessionLocal)
+
+    def test_get_db_still_yields_session(self):
+        """get_db() dependency still yields a Session (contract preserved)."""
+        from app import db  # noqa: F811
+
+        gen = db.get_db()
+        session = next(gen)
+        assert isinstance(session, db.Session)
+        session.close()
+        # Generator cleanup
+        with contextlib.suppress(StopIteration):
+            next(gen)