fix(tests): isolate plugin routes modules + redact .feedpak filenames (#736)

Two pre-existing failures the segfault had been masking (the run aborted at ~25%, so they never ran until #735 let the suite complete):

1) Tuner group (~24): plugins ship a bare-named routes.py, so sys.modules['routes'] leaked between plugin test dirs (achievements ran first, tuner got its module). Each plugin conftest now pops the stale 'routes' and an autouse fixture binds sys.modules['routes'] to that plugin's module for the duration of its tests (covers runtime 'import routes' in test bodies).

2) Diagnostics group (5): _SONG_FILENAME_RE never matched the tests' .feedpak/.archive filenames — it also lacked 'feedpak' (the current primary format), a real redaction gap. Added feedpak to the regex and switched the tests off the fake .archive to the real .feedpak. Verified: full suite 2183 passed, 0 failed.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/plugins/achievements/conftest.py

@@ -7,6 +7,8 @@ import pytest
 from fastapi import FastAPI
 from fastapi.testclient import TestClient
 
+# Drop a sibling 'routes' cached by another plugin's tests (bare-name collision).
+sys.modules.pop('routes', None)
 import routes as ach_routes
 
 
@@ -26,3 +28,17 @@ def client(tmp_path):
     app = FastAPI()
     ach_routes.setup(app, {"config_dir": str(tmp_path)})
     return TestClient(app)
+
+
+@pytest.fixture(autouse=True)
+def _bind_ach_routes():
+    """Keep sys.modules['routes'] pointing at THIS plugin's routes for these tests."""
+    prev = sys.modules.get('routes')
+    sys.modules['routes'] = ach_routes
+    try:
+        yield
+    finally:
+        if prev is not None:
+            sys.modules['routes'] = prev
+        else:
+            sys.modules.pop('routes', None)