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/handler/vote_handler.go

@@ -14,11 +14,11 @@ import (
 // VoteHandler 投票处理器
 type VoteHandler struct {
 	voteService *service.VoteService
-	postService *service.PostService
+	postService service.PostService
 }
 
 // NewVoteHandler 创建投票处理器
-func NewVoteHandler(voteService *service.VoteService, postService *service.PostService) *VoteHandler {
+func NewVoteHandler(voteService *service.VoteService, postService service.PostService) *VoteHandler {
 	return &VoteHandler{
 		voteService: voteService,
 		postService: postService,