feat(updater): add KavitaVolumeCoverUpdater for back-filling null volumes

Introduce a new background service that periodically re-checks chapters
whose volume could not be resolved at move time.

- Add KavitaVolumeCoverUpdater.py to resolve null volumes via MangaDex,
  update ComicInfo.xml in-archive, and swap in MangaBaka volume covers
- Wire updater into main.py entry point with UPDATER_ENABLED env flag
- Add UPDATER_ENABLED env var to docker-compose.prod.yml
- Update CronSchedule.py to schedule updater runs

main.py

@@ -27,6 +27,11 @@ Environment variables
     MATCH_PATH          default /config/matches.json
     WEB_PORT            default 8080  (Flask web UI for matches.json)
     WEB_HOST            default 0.0.0.0
+    UPDATER_ENABLED     default true  (volume/cover back-fill cron)
+    UPDATER_SCHEDULE    cron expression for the updater scans,
+                        default "0 19 * * 1,4" = 19:00 every Mon + Thu
+                        (local time — set TZ inside the container!)
+    UPDATER_LOG         default /config/volume_updater.log
 """
 
 from __future__ import annotations
@@ -43,6 +48,7 @@ from src.SuwayomiMover import SuwayomiMover                       # noqa: E402
 from src.SuwayomiFolderWatcher import SuwayomiFolderWatcher       # noqa: E402
 from src.MatchesCache import MatchesCache                          # noqa: E402
 from src.MatchesWebApp import MatchesWebApp                        # noqa: E402
+from src.KavitaVolumeCoverUpdater import KavitaVolumeCoverUpdater  # noqa: E402
 
 
 def _env_str(name: str, default: "str | None" = None,
@@ -85,6 +91,9 @@ def main() -> int:
     match_path      = _env_str("MATCH_PATH", "/config/matches.json")
     web_host        = _env_str("WEB_HOST", "0.0.0.0") or "0.0.0.0"
     web_port        = _env_int("WEB_PORT", 8080)
+    updater_enabled  = _env_bool("UPDATER_ENABLED", True)
+    updater_schedule = _env_str("UPDATER_SCHEDULE", "0 19 * * 1,4")
+    updater_log      = _env_str("UPDATER_LOG", "/config/volume_updater.log")
 
     print(f"[main] suwayomi  = {suwayomi_path}",  flush=True)
     print(f"[main] kavita    = {kavita_path}",    flush=True)
@@ -112,6 +121,23 @@ def main() -> int:
     web_app = MatchesWebApp(matches_cache, mover=mover, host=web_host, port=web_port)
     web_app.start()
 
+    if updater_enabled:
+        try:
+            updater = KavitaVolumeCoverUpdater(
+                kavita_path,
+                matches_cache=matches_cache,
+                language=language,
+                request_timeout=request_timeout,
+                log_path=updater_log,
+                schedule=updater_schedule,
+            )
+            updater.start()
+        except ValueError as exc:
+            # Invalid cron expression — keep the service up, just without
+            # the updater, and make the config error obvious in the logs.
+            print(f"[main] UPDATER_SCHEDULE invalid ({exc}); "
+                  f"volume/cover updater DISABLED", flush=True)
+
     # def shutdown(signum, _frame):
     #     print(f"[main] received signal {signum}", flush=True)
     #     watcher.stop()