fix: Mi2-Mi4 + Mi1 partial — version pins, file-lock, catalog lock, __main__ tests

- Mi2: requirements.txt — Obergrenzen hinzugefügt (<3.0, <5.0, etc.)
- Mi3: cmd_scan() — File-Lock via fcntl (data_dir/.scan.lock)
- Mi4: refresh_catalog() — Cache-Check unter self._lock (thread-safe)
- Mi1: tests/test_main.py — 17 neue Tests (__main__ Coverage 10% → 18%)
- 294/294 Tests grün

requirements.txt

@@ -1,5 +1,5 @@
-requests>=2.31
-beautifulsoup4>=4.12
-lxml>=5.0
-PyYAML>=6.0
-pytest>=7.0
+requests>=2.31,<3.0
+beautifulsoup4>=4.12,<5.0
+lxml>=5.0,<6.0
+PyYAML>=6.0,<7.0
+pytest>=7.0,<9.0