feat: unified OAuth token refresh API with distributed locking

Introduce OAuthRefreshAPI as the single entry point for all OAuth token
refresh operations, eliminating the race condition where background
refresh and inline refresh could simultaneously use the same
refresh_token (fixes #1035).

Key changes:
- Add OAuthRefreshExecutor interface extending TokenRefresher with CacheKey
- Add OAuthRefreshAPI.RefreshIfNeeded with lock → DB re-read → double-check flow
- Add ProviderRefreshPolicy / BackgroundRefreshPolicy strategy types
- Simplify all 4 TokenProviders to delegate to OAuthRefreshAPI
- Rewrite TokenRefreshService.refreshWithRetry to use unified API path
- Add MergeCredentials and BuildClaudeAccountCredentials helpers
- Add 40 unit tests covering all new and modified code paths

backend/internal/service/gemini_token_refresher.go

@@ -13,6 +13,11 @@ func NewGeminiTokenRefresher(geminiOAuthService *GeminiOAuthService) *GeminiToke
 	return &GeminiTokenRefresher{geminiOAuthService: geminiOAuthService}
 }
 
+// CacheKey 返回用于分布式锁的缓存键
+func (r *GeminiTokenRefresher) CacheKey(account *Account) string {
+	return GeminiTokenCacheKey(account)
+}
+
 func (r *GeminiTokenRefresher) CanRefresh(account *Account) bool {
 	return account.Platform == PlatformGemini && account.Type == AccountTypeOAuth
 }
@@ -35,11 +40,7 @@ func (r *GeminiTokenRefresher) Refresh(ctx context.Context, account *Account) (m
 	}
 
 	newCredentials := r.geminiOAuthService.BuildAccountCredentials(tokenInfo)
-	for k, v := range account.Credentials {
-		if _, exists := newCredentials[k]; !exists {
-			newCredentials[k] = v
-		}
-	}
+	newCredentials = MergeCredentials(account.Credentials, newCredentials)
 
 	return newCredentials, nil
 }