refactor(oauth): replace legacy oauth cache with standard ram cache and add pubsub synchronization

- Replaced custom map-based cache in apps/oauth/cache.go with standard pkg/cache/ram framework.
- Implemented Redis Pub/Sub invalidation channels for distributed token and user cache synchronization.
- Created apps/oauth/cache_test.go to verify local cache operations and pub/sub broadcasts.

refactor(cache): generic RAM cache with CoW and unified preheating

Replaced L2 Redis cache and old cache package with process-local generic pkg/cache/ram. Implemented Copy-on-Write for reads, fine-grained locks per type for writes, and unified preheating in bootstrap. Changed cache invalidation to lazy-loading to resolve SQLite deadlocks during transactions.

internal/repository/user.go

@@ -32,12 +32,12 @@ func GetUserByUsername(ctx context.Context, username string) (model.User, error)
 // GetSystemUser loads the built-in system user, or returns a synthetic fallback.
 func GetSystemUser(ctx context.Context) model.User {
 	var user model.User
-	if err := db.DB(ctx).Where("username = ?", "system").First(&user).Error; err == nil {
+	if err := db.DB(ctx).Where("username = ?", configTypeSystem).First(&user).Error; err == nil {
 		return user
 	}
 	return model.User{
 		ID:       999,
-		Username: "system",
+		Username: configTypeSystem,
 		Nickname: "系统",
 	}
 }