chore: restore sw_threading requirements.txt and ignore *.txt in license check

The plugin compose file installs /app/requirements.txt, so deleting it broke
the sw_threading test. Bring it back and exclude *.txt from the license
header check instead — per-plugin requirements.txt files are generated by
tests/plugin/conftest.py and can never carry a header.

Author: kezhenxu94

.licenserc.yaml

@@ -32,5 +32,6 @@ header:
     - '**/*.json'
     - '.venv'
     - 'poetry.lock'
+    - '**/*.txt'
 
   comment: on-failure

tests/plugin/web/sw_threading/requirements.txt

@@ -0,0 +1 @@
+