fix: add i18n keys, lazy logging, and improve tests per contributor guide

- Add storage.breakdown.* i18n keys to en/views/system.json locale file
  and remove inline fallback strings from t() calls
- Use lazy logging format (%s) instead of f-string in new debug log line
- Rewrite retain_policy tests to target RecordConfig directly, avoiding
  Docker-only dependencies (labelmap, /config, detector runtimes)
- Add enum value and serialization roundtrip tests

web/public/locales/en/views/system.json

@@ -110,6 +110,11 @@
       "title": "SHM (shared memory) allocation",
       "warning": "The current SHM size of {{total}}MB is too small. Increase it to at least {{min_shm}}MB."
     },
+    "breakdown": {
+      "overwritable": "Continuous (overwritable)",
+      "eventRetention": "Events (aging out)",
+      "protected": "Protected (indefinite)"
+    },
     "cameraStorage": {
       "title": "Camera Storage",
       "camera": "Camera",