feat(etl-cache): add EvictionCandidate value object

surfsense_backend/app/etl_pipeline/cache/schemas/eviction_candidate.py

@@ -0,0 +1,15 @@
+"""Row projection handed to the eviction policy."""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+from datetime import datetime
+
+
+@dataclass(frozen=True, slots=True)
+class EvictionCandidate:
+    id: int
+    storage_key: str
+    size_bytes: int
+    last_used_at: datetime
+    times_reused: int