refactor: extract eviction engine into eviction.py (rag.py 303→109 lines)

- Move all eviction logic (evict_batch, maybe_evict, EVICTION_LOG,
  get_collection_count/stats, get_rag_operational_stats) into eviction.py
- Move QDRANT_URL, RAG_COLLECTION into config.py to break circular dep
- rag.py re-exports eviction symbols for backward compatibility
- Router imports updated to use eviction module directly
- All 130 tests pass

routers/rag_admin.py

@@ -5,10 +5,8 @@ import httpx
 from fastapi import APIRouter, Request
 from fastapi.responses import JSONResponse
 
-from rag import (
-    get_rag_operational_stats, EVICTION_LOG,
-    QDRANT_URL, RAG_COLLECTION,
-)
+from eviction import get_rag_operational_stats, EVICTION_LOG
+from rag import QDRANT_URL, RAG_COLLECTION
 
 log = logging.getLogger("jarvischat")
 router = APIRouter()