Server.running was a plain bool accessed from multiple goroutines
(start/stop/signal handlers). Convert to atomic.Bool with
Store/Load to make all accesses safe for concurrent use.
Updates all test files to use the new atomic API.
Remove unused disk cache, tiered cache, purge, and config loader code.
Add HashPathWithMethod and MatchPattern helpers for future cache purge API.
Update test to use new mock backend API with ResponseBody field.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>