Add token usage stats and consolidate credentials in SQLite

- New internal/stats (types) and internal/store (SQLite owner: requests + credentials tables, WAL); store implements stats.Recorder.
- Stream (SSE tee) and non-stream chat paths parse upstream usage incl. cached_tokens and record per-request; add /api/stats, /api/stats/reset, /admin/stats HTML with cache hit rate.
- Drop credentials.json: remove auth file I/O and ZHANLU_CREDENTIALS_FILE; credential precedence is env vars > db row.

.gitignore

@@ -1,4 +1,9 @@
-credentials.json
+zhanlu.db
+zhanlu.db-shm
+zhanlu.db-wal
+*.db
+*.db-shm
+*.db-wal
 extension/
 *.exe
 *.log