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.

cmd/zhanlu-proxy/main.go

@@ -6,6 +6,7 @@ import (
 
 	"git.misaka.ren/M1saka/zhanlu_proxy/internal/config"
 	"git.misaka.ren/M1saka/zhanlu_proxy/internal/server"
+	"git.misaka.ren/M1saka/zhanlu_proxy/internal/store"
 )
 
 func main() {
@@ -14,7 +15,20 @@ func main() {
 		log.Fatal(err)
 	}
 
-	h := server.New(cfg)
+	st, err := store.Open(cfg.DBPath)
+	if err != nil {
+		log.Fatalf("open store: %v", err)
+	}
+	defer st.Close()
+
+	// Credential precedence at startup: environment variables > db row.
+	if cfg.Credentials.Validate() != nil && !cfg.Credentials.HasAPIKey() {
+		if dbCreds, derr := st.LoadCredentials(); derr == nil && (dbCreds.HasAPIKey() || dbCreds.Validate() == nil) {
+			cfg.Credentials = dbCreds
+		}
+	}
+
+	h := server.New(cfg, st)
 	log.Printf("zhanlu proxy listening on %s", cfg.ListenAddr)
 	log.Printf("login page: http://127.0.0.1%s/login", cfg.ListenAddr)
 	if err := http.ListenAndServe(cfg.ListenAddr, h); err != nil {