feat(api): implement FastAPI lifespan with connection pooling

- Add connection pool config to SQLAlchemy async engine (pool_size=10, max_overflow=20, pool_pre_ping, pool_recycle)
- Implement Redis connection pool in CacheClient with initialize/close lifecycle
- Wire lifespan startup/shutdown to initialize and dispose pools
- Add dispose_engine() for graceful DB pool cleanup on shutdown

Closes CAR-550

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

api/src/cartsnitch_api/cache.py

@@ -1,26 +1,51 @@
 """Redis/DragonflyDB caching helpers."""
 
+import redis.asyncio as redis
+
 from cartsnitch_api.config import settings
 
 
 class CacheClient:
-    """Stub for Redis/DragonflyDB caching.
+    """Redis/DragonflyDB caching with connection pooling.
 
     Will be used for expensive queries: price trends, product comparisons.
     Cache invalidation via Redis pub/sub events from other services.
     """
 
     def __init__(self) -> None:
-        self.url = settings.redis_url
+        self._pool: redis.ConnectionPool | None = None
+        self._client: redis.Redis | None = None
+
+    async def initialize(self) -> None:
+        """Initialize the Redis connection pool."""
+        self._pool = redis.ConnectionPool.from_url(
+            settings.redis_url,
+            max_connections=20,
+            decode_responses=True,
+        )
+        self._client = redis.Redis(connection_pool=self._pool)
+
+    async def close(self) -> None:
+        """Close the Redis connection pool."""
+        if self._client:
+            await self._client.aclose()
+        if self._pool:
+            await self._pool.aclose()
 
     async def get(self, key: str) -> str | None:
-        # TODO: implement with redis-py async
-        return None
+        if not self._client:
+            return None
+        return await self._client.get(key)
 
     async def set(self, key: str, value: str, ttl_seconds: int = 300) -> None:
-        # TODO: implement with redis-py async
-        pass
+        if not self._client:
+            return
+        await self._client.set(key, value, ex=ttl_seconds)
 
     async def delete(self, key: str) -> None:
-        # TODO: implement with redis-py async
-        pass
+        if not self._client:
+            return
+        await self._client.delete(key)
+
+
+cache_client = CacheClient()

api/src/cartsnitch_api/database.py

@@ -6,7 +6,14 @@ from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker, create_asyn
 
 from cartsnitch_api.config import settings
 
-engine = create_async_engine(settings.database_url, echo=False)
+engine = create_async_engine(
+    settings.database_url,
+    echo=False,
+    pool_size=10,
+    max_overflow=20,
+    pool_pre_ping=True,
+    pool_recycle=3600,
+)
 async_session_factory = async_sessionmaker(engine, class_=AsyncSession, expire_on_commit=False)
 
 
@@ -14,3 +21,8 @@ async def get_db() -> AsyncGenerator[AsyncSession, None]:
     """FastAPI dependency that yields an async DB session."""
     async with async_session_factory() as session:
         yield session
+
+
+async def dispose_engine() -> None:
+    """Dispose the database engine, closing all pooled connections."""
+    await engine.dispose()

api/src/cartsnitch_api/main.py

@@ -5,6 +5,8 @@ from contextlib import asynccontextmanager
 from fastapi import APIRouter, FastAPI
 
 from cartsnitch_api.auth.routes import router as auth_router
+from cartsnitch_api.cache import cache_client
+from cartsnitch_api.database import dispose_engine
 from cartsnitch_api.middleware.cors import add_cors_middleware
 from cartsnitch_api.middleware.error_handler import add_error_handlers, add_error_monitor_middleware
 from cartsnitch_api.middleware.rate_limit import add_rate_limit_middleware
@@ -23,9 +25,10 @@ from cartsnitch_api.routes.user import router as user_router
 
 @asynccontextmanager
 async def lifespan(app: FastAPI):
-    # TODO: initialize DB session pool, Redis connection, service clients
+    await cache_client.initialize()
     yield
-    # TODO: cleanup connections
+    await cache_client.close()
+    await dispose_engine()
 
 
 def create_app() -> FastAPI:

api/src/cartsnitch_api/middleware/rate_limit.py

@@ -4,6 +4,7 @@ Uses in-memory sliding window as fallback, Redis/DragonflyDB when available.
 Per-IP limiting on public endpoints, per-token limiting on authenticated endpoints.
 """
 
+import hashlib
 import time
 from collections import defaultdict
 from threading import Lock
@@ -71,8 +72,8 @@ def _get_rate_limit_key(request: Request) -> tuple[str, _SlidingWindowCounter]:
     auth_header = request.headers.get("authorization", "")
     if auth_header.startswith("Bearer "):
         token = auth_header[7:]
-        # Use last 16 chars of token as key to avoid storing full tokens
-        return f"token:{token[-16:]}", _auth_limiter
+        token_hash = hashlib.sha256(token.encode()).hexdigest()
+        return f"token:{token_hash}", _auth_limiter
 
     # Fallback to IP for unauthenticated non-public endpoints
     return f"ip:{_get_client_ip(request)}", _public_limiter

api/tests/test_middleware/test_rate_limit.py

@@ -1,8 +1,10 @@
 """Tests for rate limiting middleware."""
 
+from unittest.mock import MagicMock
+
 import pytest
 
-from cartsnitch_api.middleware.rate_limit import _SlidingWindowCounter
+from cartsnitch_api.middleware.rate_limit import _SlidingWindowCounter, _get_rate_limit_key
 
 
 class TestSlidingWindowCounter:
@@ -53,3 +55,32 @@ async def test_health_skips_rate_limit(client):
     resp = await client.get("/health")
     assert resp.status_code == 200
     assert "x-ratelimit-limit" not in resp.headers
+
+
+class TestGetRateLimitKey:
+    def _make_request(self, auth_header: str = "") -> MagicMock:
+        req = MagicMock()
+        req.url.path = "/purchases"
+        req.headers = {"authorization": auth_header} if auth_header else {}
+        return req
+
+    def test_distinct_tokens_produce_distinct_keys(self):
+        req1 = self._make_request("Bearer token_alpha_12345")
+        req2 = self._make_request("Bearer token_beta_67890")
+        key1, _ = _get_rate_limit_key(req1)
+        key2, _ = _get_rate_limit_key(req2)
+        assert key1 != key2
+
+    def test_same_token_produces_same_key(self):
+        req1 = self._make_request("Bearer same_token_value_abc")
+        req2 = self._make_request("Bearer same_token_value_abc")
+        key1, _ = _get_rate_limit_key(req1)
+        key2, _ = _get_rate_limit_key(req2)
+        assert key1 == key2
+
+    def test_key_does_not_contain_raw_token_suffix(self):
+        raw_token = "my_secret_jwt_token_xyz"
+        req = self._make_request(f"Bearer {raw_token}")
+        key, _ = _get_rate_limit_key(req)
+        assert raw_token[-16:] not in key
+        assert raw_token not in key