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

config.py

@@ -50,6 +50,8 @@ BODY_LIMIT_PROFILE_BYTES = 256 * 1024
 UPLOAD_DIR = "/tmp/jarvischat_uploads"
 MAX_UPLOAD_BYTES = 20 * 1024 * 1024
 SUPPORTED_UPLOAD_TYPES = {"text/plain", "text/markdown", "application/pdf", "application/json", "text/x-python", "text/html", "image/png", "image/jpeg", "image/gif", "image/svg+xml", "image/webp"}
+QDRANT_URL = "http://192.168.50.108:6333"
+RAG_COLLECTION = "jarvis_rag"
 UPLOAD_CONTEXT_EXPIRY_HOURS = 1
 BODY_LIMIT_UPLOAD_BYTES = MAX_UPLOAD_BYTES