feat(repository): 实现 Gemini OAuth 和 Token 缓存客户端

- 添加 Gemini OAuth 客户端实现
- 实现 Redis 基础的 Token 缓存
- 添加 gemini-cli Code Assist 客户端封装

backend/internal/repository/gemini_token_cache.go

@@ -0,0 +1,44 @@
+package repository
+
+import (
+	"context"
+	"fmt"
+	"time"
+
+	"github.com/Wei-Shaw/sub2api/internal/service/ports"
+
+	"github.com/redis/go-redis/v9"
+)
+
+const (
+	geminiTokenKeyPrefix       = "gemini:token:"
+	geminiRefreshLockKeyPrefix = "gemini:refresh_lock:"
+)
+
+type geminiTokenCache struct {
+	rdb *redis.Client
+}
+
+func NewGeminiTokenCache(rdb *redis.Client) ports.GeminiTokenCache {
+	return &geminiTokenCache{rdb: rdb}
+}
+
+func (c *geminiTokenCache) GetAccessToken(ctx context.Context, cacheKey string) (string, error) {
+	key := fmt.Sprintf("%s%s", geminiTokenKeyPrefix, cacheKey)
+	return c.rdb.Get(ctx, key).Result()
+}
+
+func (c *geminiTokenCache) SetAccessToken(ctx context.Context, cacheKey string, token string, ttl time.Duration) error {
+	key := fmt.Sprintf("%s%s", geminiTokenKeyPrefix, cacheKey)
+	return c.rdb.Set(ctx, key, token, ttl).Err()
+}
+
+func (c *geminiTokenCache) AcquireRefreshLock(ctx context.Context, cacheKey string, ttl time.Duration) (bool, error) {
+	key := fmt.Sprintf("%s%s", geminiRefreshLockKeyPrefix, cacheKey)
+	return c.rdb.SetNX(ctx, key, 1, ttl).Result()
+}
+
+func (c *geminiTokenCache) ReleaseRefreshLock(ctx context.Context, cacheKey string) error {
+	key := fmt.Sprintf("%s%s", geminiRefreshLockKeyPrefix, cacheKey)
+	return c.rdb.Del(ctx, key).Err()
+}