fix(receiptwitness): pool DB engine and Redis client to prevent connection exhaustion

email_worker calls get_async_session_factory() inside every resolve_user()
call, which creates a brand-new async engine (and thus a brand-new
connection pool) on every message.  In a tight consumer loop processing
5 messages per batch, this rapidly exhausts DragonflyDB/Postgres
connection limits and manifests as ConnectionResetError.

Fix: cache the async engine in a module-level dict keyed by URL in
cartsnitch_common.database:get_async_engine(), matching the pattern
already used in receiptwitness:events.py for the Redis connection pool.
Also add pool_size=10, max_overflow=20, pool_pre_ping=True for
健壮连接管理.

Similarly, receiptwitness/queue/email.py:get_redis() was creating a new
Redis connection on every call with no pooling.  Share a
ConnectionPool (max_connections=30) across all get_redis() callers.

Fixes CAR-1078
Co-Authored-By: Paperclip <noreply@paperclip.ing>

receiptwitness/src/receiptwitness/queue/email.py

@@ -16,6 +16,29 @@ logger = logging.getLogger(__name__)
 STREAM_KEY = "email:receipts"
 CONSUMER_GROUP = "email-workers"
 
+# Module-level Redis/DragonflyDB connection pool — shared across all worker calls.
+# Without pooling, each call to get_redis() opens a new TCP connection.  In a tight
+# consumer loop this causes ConnectionResetError when DragonflyDB's connection limit
+# is hit under load.  max_connections=30 (10 base + 20 overflow) mirrors the engine pool.
+_redis_pool: aioredis.ConnectionPool | None = None
+
+
+def _get_redis_pool() -> aioredis.ConnectionPool:
+    """Get or create the shared DragonflyDB connection pool."""
+    global _redis_pool
+    if _redis_pool is None:
+        _redis_pool = aioredis.ConnectionPool.from_url(
+            settings.redis_url,
+            decode_responses=True,
+            max_connections=30,
+        )
+    return _redis_pool
+
+
+async def get_redis() -> aioredis.Redis:
+    """Get async Redis/DragonflyDB client backed by a shared connection pool."""
+    return aioredis.Redis(connection_pool=_get_redis_pool())
+
 
 @dataclass
 class EmailJob:
@@ -31,11 +54,6 @@ class EmailJob:
     message_id: str  # from email provider, for dedup
 
 
-async def get_redis() -> aioredis.Redis:
-    """Get async Redis/DragonflyDB client."""
-    return cast(aioredis.Redis, aioredis.from_url(settings.redis_url, decode_responses=True))
-
-
 async def ensure_consumer_group(client: aioredis.Redis) -> None:
     """Create consumer group if it does not exist."""
     try: