feat(etl-cache): add eviction policy

surfsense_backend/app/etl_pipeline/cache/eviction/policy.py

@@ -0,0 +1,28 @@
+"""Pure selection rules for which cached entries to drop."""
+
+from __future__ import annotations
+
+from collections.abc import Iterable
+
+from app.etl_pipeline.cache.schemas import EvictionCandidate
+
+
+def select_over_budget(
+    coldest_first: Iterable[EvictionCandidate],
+    *,
+    current_total_bytes: int,
+    max_total_bytes: int,
+) -> list[EvictionCandidate]:
+    """Pick coldest entries until the footprint drops under the budget."""
+    bytes_to_free = current_total_bytes - max_total_bytes
+    if bytes_to_free <= 0:
+        return []
+
+    chosen: list[EvictionCandidate] = []
+    bytes_freed = 0
+    for candidate in coldest_first:
+        if bytes_freed >= bytes_to_free:
+            break
+        chosen.append(candidate)
+        bytes_freed += candidate.size_bytes
+    return chosen