feat(api): implement lifespan with DB and Redis connection pooling

- Refactor database.py to use init_db()/close_db() lifecycle
- Add create_db_engine() with pool_size=10, max_overflow=20, pool_pre_ping=True
- Replace cache.py stub with real Redis client using redis.asyncio
- Implement init_redis()/close_redis() with graceful error handling
- Replace no-op lifespan in main.py with proper startup/shutdown
- Enhance health endpoint to check DB and Redis connectivity
- Add tests for database, cache, and health endpoint lifecycle

Co-Authored-By: Paperclip <noreply@paperclip.ing>

api/tests/test_cache.py

@@ -0,0 +1,50 @@
+"""Tests for Redis/DragonflyDB caching lifecycle."""
+
+import pytest
+
+from cartsnitch_api.cache import CacheClient, close_redis, get_redis, init_redis
+
+
+@pytest.mark.asyncio
+async def test_init_redis_creates_client():
+    """Test that init_redis creates the Redis client."""
+    await init_redis()
+    try:
+        r = get_redis()
+        assert r is not None
+        await r.ping()
+    finally:
+        await close_redis()
+
+
+@pytest.mark.asyncio
+async def test_close_redis_clears_client():
+    """Test that close_redis properly closes and clears the client."""
+    await init_redis()
+    await close_redis()
+    assert get_redis() is None
+
+
+@pytest.mark.asyncio
+async def test_cache_client_get_returns_none_when_not_connected():
+    """Test that CacheClient.get returns None gracefully when Redis is down."""
+    client = CacheClient()
+    # Without init_redis, get should return None
+    result = await client.get("test-key")
+    assert result is None
+
+
+@pytest.mark.asyncio
+async def test_cache_client_set_does_not_raise_when_not_connected():
+    """Test that CacheClient.set does not raise when Redis is down."""
+    client = CacheClient()
+    # Without init_redis, set should not raise
+    await client.set("test-key", "test-value", ttl_seconds=60)
+
+
+@pytest.mark.asyncio
+async def test_cache_client_delete_does_not_raise_when_not_connected():
+    """Test that CacheClient.delete does not raise when Redis is down."""
+    client = CacheClient()
+    # Without init_redis, delete should not raise
+    await client.delete("test-key")

api/tests/test_database.py

@@ -0,0 +1,62 @@
+"""Tests for database initialization and lifecycle."""
+
+import pytest
+from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker, create_async_engine
+
+from cartsnitch_api.database import (
+    close_db,
+    create_db_engine,
+    get_engine,
+    init_db,
+)
+
+
+@pytest.mark.asyncio
+async def test_create_db_engine_creates_engine_with_pool_settings():
+    """Test that create_db_engine creates engine with correct pool settings."""
+    engine = create_db_engine()
+    assert engine is not None
+    pool = engine.pool
+    assert pool.size() == 10
+    assert pool._max_overflow == 20
+    await engine.dispose()
+
+
+@pytest.mark.asyncio
+async def test_init_db_sets_engine_and_factory():
+    """Test that init_db properly initializes the engine and session factory."""
+    await init_db()
+    try:
+        eng = get_engine()
+        assert eng is not None
+        from cartsnitch_api import database
+
+        assert database.async_session_factory is not None
+    finally:
+        await close_db()
+
+
+@pytest.mark.asyncio
+async def test_close_db_disposes_engine():
+    """Test that close_db properly disposes the engine."""
+    await init_db()
+    await close_db()
+    assert get_engine() is None
+    from cartsnitch_api import database
+
+    assert database.async_session_factory is None
+
+
+@pytest.mark.asyncio
+async def test_get_db_yields_session_after_init():
+    """Test that get_db yields working sessions after init_db."""
+    await init_db()
+    try:
+        from cartsnitch_api.database import get_db
+
+        gen = get_db()
+        session = await gen.__anext__()
+        assert isinstance(session, AsyncSession)
+        await gen.aclose()
+    finally:
+        await close_db()

api/tests/test_routes/test_health.py

@@ -0,0 +1,77 @@
+"""Tests for health check endpoint."""
+
+import pytest
+from unittest.mock import AsyncMock, patch
+
+from cartsnitch_api.database import init_db, close_db
+
+
+@pytest.mark.asyncio
+async def test_health_returns_db_and_redis_fields(client):
+    """Test that health endpoint returns db and redis status fields."""
+    from cartsnitch_api.cache import init_redis, close_redis
+
+    await init_db()
+    await init_redis()
+
+    try:
+        response = await client.get("/health")
+        assert response.status_code == 200
+        data = response.json()
+        assert "status" in data
+        assert "db" in data
+        assert "redis" in data
+    finally:
+        await close_redis()
+        await close_db()
+
+
+@pytest.mark.asyncio
+async def test_health_returns_degraded_when_db_down():
+    """Test that health returns degraded when database is down."""
+    from cartsnitch_api.database import _engine
+    from cartsnitch_api.routes.health import health
+
+    # Simulate engine is None (DB not initialized)
+    with patch("cartsnitch_api.routes.health.get_engine", return_value=None):
+        response = await health()
+        assert response["status"] == "degraded"
+        assert response["db"] is False
+
+
+@pytest.mark.asyncio
+async def test_health_returns_ok_when_db_up(client):
+    """Test that health returns ok when database is up."""
+    from cartsnitch_api.database import init_db, close_db
+    from cartsnitch_api.cache import init_redis, close_redis
+
+    await init_db()
+    await init_redis()
+
+    try:
+        response = await client.get("/health")
+        assert response.status_code == 200
+        data = response.json()
+        if data["db"]:
+            assert data["status"] == "ok"
+    finally:
+        await close_redis()
+        await close_db()
+
+
+@pytest.mark.asyncio
+async def test_health_redis_down_does_not_make_unhealthy(client):
+    """Test that Redis being down does not make health return unhealthy."""
+    from cartsnitch_api.database import init_db, close_db
+
+    await init_db()
+
+    try:
+        response = await client.get("/health")
+        data = response.json()
+        # Redis being down should not make status "degraded"
+        # Only DB failure makes it degraded
+        if not data["db"]:
+            assert data["status"] == "degraded"
+    finally:
+        await close_db()