Add --debug flag, dual-mode logging, and auto-run on startup

- logger.py: root-level handlers; normal mode shows only user-facing INFO
  (sync, export, push, MBOX) plus WARNING/ERROR; --debug shows all DEBUG
  with full context (module.func:line). Third-party loggers silenced to WARNING.
- main.py: add --debug CLI flag, call configure_logging() at startup,
  auto-trigger sync on first run or when last sync is overdue by the interval
- database.py: add metadata table with record_sync_time() / get_last_sync_time()
  so startup knows whether a sync is due; sync time recorded on success
- forwarder.py: INFO at push start and push complete with counts
- packager.py: INFO before MBOX conversion begins
- exporter.py: INFO when Proton Mail export starts

https://claude.ai/code/session_01KjaNo9RXevw6x1DjJD8mj6

modules/exporter.py

@@ -73,6 +73,8 @@ def run_export(
     out_dir = export_dir or EXPORT_DIR
     out_dir.mkdir(parents=True, exist_ok=True)
 
+    log.info("Starting Proton Mail export...")
+
     try:
         binary = ensure_export_cli()
     except Exception as exc: