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.

internal/auth/credentials.go

@@ -1,10 +1,7 @@
 package auth
 
 import (
-	"encoding/json"
 	"errors"
-	"os"
-	"path/filepath"
 	"strings"
 	"time"
 )
@@ -49,30 +46,3 @@ func (c Credentials) Validate() error {
 func (c Credentials) HasAPIKey() bool {
 	return strings.TrimSpace(c.APIKey) != "" && strings.TrimSpace(c.ModelBaseURL) != ""
 }
-
-func LoadCredentials(path string) (Credentials, error) {
-	b, err := os.ReadFile(path)
-	if err != nil {
-		return Credentials{}, err
-	}
-	var c Credentials
-	if err := json.Unmarshal(b, &c); err != nil {
-		return Credentials{}, err
-	}
-	return c, c.Validate()
-}
-
-func SaveCredentials(path string, c Credentials) error {
-	if err := c.Validate(); err != nil {
-		return err
-	}
-	c.SavedAt = time.Now()
-	b, err := json.MarshalIndent(c, "", "  ")
-	if err != nil {
-		return err
-	}
-	if err := os.MkdirAll(filepath.Dir(path), 0o700); err != nil {
-		return err
-	}
-	return os.WriteFile(path, b, 0o600)
-}