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.

.gitignore

@@ -7,6 +7,9 @@
 # python cache
 __pycache__/
 
+# incremental-scan state (regenerated each run)
+data/scan_state.json
+
 # reports data
 reports/copyparty/
 reports/mikrotik/