feat(etl-cache): add cache blob object-key builder

surfsense_backend/app/etl_pipeline/cache/storage/object_keys.py

@@ -0,0 +1,12 @@
+"""Object keys for cached markdown, namespaced under a dedicated prefix."""
+
+from __future__ import annotations
+
+from app.etl_pipeline.cache.schemas import ParseKey
+
+CACHE_PREFIX = "etl_cache"
+
+
+def build_parse_object_key(key: ParseKey) -> str:
+    # Content-addressed: identical bytes + recipe always map to the same key.
+    return f"{CACHE_PREFIX}/{key.source_sha256}/{key.object_suffix}"