feat: add incremental log scanning with resumable byte offsets

Track last processed byte offset per log file so scans only read new
entries, add --full flag to force a rescan, and persist state atomically.

app/web.py

@@ -102,6 +102,7 @@ def _params_from_request() -> ScanParams:
         report_dir=str(_pick("report_dir", REPORT_DIR)),
         top=int(_pick("top", DEFAULT_TOP)),
         quiet=bool(data.get("quiet", False)),
+        full=bool(data.get("full", False)),
         update_firewall=bool(data.get("update_firewall", False)),
         dry_run=bool(data.get("dry_run", False)),
     )