feat(B-RED): Zentraler Redis Pool — cache.py, monitoring.py, worker.py auf get_redis() umgestellt

B-RED: 5 direkte aioredis.from_url() Konstruktoren auf get_redis() umgestellt
- cache.py: get_cache() delegiert auf get_redis(), _cache_redis Singleton entfernt
- monitoring.py: check_redis() und check_worker() nutzen get_redis()
- worker.py: _acquire_cron_lock() und _release_cron_lock() nutzen get_redis()
- 0 verbleibende direkte aioredis.from_url() außerhalb auth.py

B-RED-TEST: 8 Tests in test_redis_pool.py — alle grün
- Singleton, get_cache delegation, SET/GET, parallel, reset, no-direct-from_url checks

app/core/monitoring.py

@@ -149,14 +149,10 @@ async def check_database() -> dict[str, Any]:
 async def check_redis() -> dict[str, Any]:
     """Check Redis connectivity (async)."""
     try:
-        import redis.asyncio as aioredis
+        from app.core.auth import get_redis
 
-        from app.config import get_settings
-
-        settings = get_settings()
-        r = aioredis.from_url(settings.redis_url, decode_responses=True)
+        r = get_redis()
         pong = await r.ping()
-        await r.aclose()
         if pong:
             return {"status": "up", "latency_ms": 0}
         return {"status": "down", "error": "Redis returned False for PING"}
@@ -185,15 +181,11 @@ async def check_worker() -> dict[str, Any]:
     In test/dev mode this checks if Redis is available for the worker queue.
     """
     try:
-        import redis.asyncio as aioredis
+        from app.core.auth import get_redis
 
-        from app.config import get_settings
-
-        settings = get_settings()
-        r = aioredis.from_url(settings.redis_url, decode_responses=True)
+        r = get_redis()
         # Check if arq queue key exists
         queue_length = await r.zcard("arq:queue")
-        await r.aclose()
         return {"status": "up", "queue_length": queue_length}
     except Exception as e:
         return {"status": "down", "error": str(e)}