Fix deprecated fitz import, log unhandled scan-thread crashes

document_scanner.py: `import fitz` is PyMuPDF's deprecated import
alias, scheduled for removal - switched both sites to `import pymupdf
as _fitz`. Pure rename, no behavior change (all 227 tests still pass).

routes/scan.py: the background scan thread caught no exceptions, so
an unexpected crash in run_scan() escaped into Python's default
unhandled-thread-exception hook instead of the app's own logging/audit
trail. The scan lock was already released correctly via `finally` -
now the crash itself is also logged (`_log.exception`, full traceback)
and audit-recorded before that. Confirmed by test_lock_released_when_
run_scan_raises: the PytestUnhandledThreadExceptionWarning it
triggered is now gone, with no change to the test's own assertions.

routes/scan.py

@@ -114,6 +114,9 @@ def scan_start():
         try:
             run_scan(options)
             _maybe_send_auto_email()
+        except Exception:
+            _log.exception("[scan] background scan thread crashed")
+            _audit("scan_error", "background scan thread crashed", ip="")
         finally:
             state._scan_lock.release()
     threading.Thread(target=_run, daemon=True).start()