feat(tests): Phase 0 - Fix flaky and broken tests

Implements Phase 0 of v1.5.0 per ADR-012 and RELEASE.md.

Changes:
- Remove 5 broken multiprocessing tests (TestConcurrentExecution, TestPerformance)
- Fix brittle XML assertion tests (check semantics not quote style)
- Fix test_debug_level_for_early_retries logger configuration
- Rename test_feed_route_streaming to test_feed_route_caching (correct name)

Results:
- Test count: 879 → 874 (5 removed as planned)
- All tests pass consistently (verified across 3 runs)
- No flakiness detected

References:
- ADR-012: Flaky Test Removal and Test Quality Standards
- docs/projectplan/v1.5.0/RELEASE.md Phase 0

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

tests/test_routes_feed.py

@@ -114,7 +114,7 @@ class TestFeedRoute:
         cache_seconds = app.config.get("FEED_CACHE_SECONDS", 300)
         assert f"max-age={cache_seconds}" in response.headers["Cache-Control"]
 
-    def test_feed_route_streaming(self, client):
+    def test_feed_route_caching(self, client):
         """Test /feed.xml uses cached response (with ETag)"""
         response = client.get("/feed.xml")
         assert response.status_code == 200