refactor: introduce Wire dependency injection and interface-based architecture

- Add Google Wire for compile-time dependency injection
- Replace concrete service types with interfaces across handlers
- Remove global state (DB, cache) in favor of constructor injection
- Split monolithic files into focused modules:
  - config: separate files for each config domain
  - dto: converters split by domain (user, post, message, group)
  - cache: separate metrics.go and redis_cache.go
- Introduce unified apperrors package with string-based error codes
- Add transaction support with context-aware repository methods
- Upgrade golang.org/x/crypto from 0.17.0 to 0.26.0

internal/config/external.go

@@ -0,0 +1,24 @@
+package config
+
+// GorseConfig Gorse 推荐系统配置
+type GorseConfig struct {
+	Address         string `mapstructure:"address"`
+	APIKey          string `mapstructure:"api_key"`
+	Enabled         bool   `mapstructure:"enabled"`
+	Dashboard       string `mapstructure:"dashboard"`
+	ImportPassword  string `mapstructure:"import_password"`
+	EmbeddingAPIKey string `mapstructure:"embedding_api_key"`
+	EmbeddingURL    string `mapstructure:"embedding_url"`
+	EmbeddingModel  string `mapstructure:"embedding_model"`
+}
+
+// OpenAIConfig OpenAI 配置
+type OpenAIConfig struct {
+	Enabled                       bool   `mapstructure:"enabled"`
+	BaseURL                       string `mapstructure:"base_url"`
+	APIKey                        string `mapstructure:"api_key"`
+	ModerationModel               string `mapstructure:"moderation_model"`
+	ModerationMaxImagesPerRequest int    `mapstructure:"moderation_max_images_per_request"`
+	RequestTimeout                int    `mapstructure:"request_timeout"`
+	StrictModeration              bool   `mapstructure:"strict_moderation"`
+}