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_scan.py

@@ -100,6 +100,7 @@ def start_scan(kind: ScanKind, params: ScanParams) -> dict[str, Any]:
                     report_dir=params.copyparty_report_dir,
                     top=params.top,
                     quiet=params.quiet,
+                    full=params.full,
                     min_denied=params.min_denied,
                 ).run()
                 job.result = ScanResult(ok=True)
@@ -111,6 +112,7 @@ def start_scan(kind: ScanKind, params: ScanParams) -> dict[str, Any]:
                     report_dir=params.npm_report_dir,
                     top=params.top,
                     quiet=params.quiet,
+                    full=params.full,
                     min_errors=params.min_errors,
                     error_ratio=params.error_ratio,
                 ).run()