refactor: Introduce pre-consume quota and unify relay handlers

This commit introduces a major architectural refactoring to improve quota management, centralize logging, and streamline the relay handling logic.

Key changes:
- **Pre-consume Quota:** Implements a new mechanism to check and reserve user quota *before* making the request to the upstream provider. This ensures more accurate quota deduction and prevents users from exceeding their limits due to concurrent requests.

- **Unified Relay Handlers:** Refactors the relay logic to use generic handlers (e.g., `ChatHandler`, `ImageHandler`) instead of provider-specific implementations. This significantly reduces code duplication and simplifies adding new channels.

- **Centralized Logger:** A new dedicated `logger` package is introduced, and all system logging calls are migrated to use it, moving this responsibility out of the `common` package.

- **Code Reorganization:** DTOs are generalized (e.g., `dalle.go` -> `openai_image.go`) and utility code is moved to more appropriate packages (e.g., `common/http.go` -> `service/http.go`) for better code structure.

types/relay_request.go

@@ -0,0 +1,27 @@
+package types
+
+type RelayRequest struct {
+	OriginRequest    any
+	Format           RelayFormat
+	PromptTokenCount int
+}
+
+func (r *RelayRequest) CopyOriginRequest() any {
+	if r.OriginRequest == nil {
+		return nil
+	}
+	switch v := r.OriginRequest.(type) {
+	case *GeneralOpenAIRequest:
+		return v.Copy()
+	case *GeneralClaudeRequest:
+		return v.Copy()
+	case *GeneralGeminiRequest:
+		return v.Copy()
+	case *GeneralRerankRequest:
+		return v.Copy()
+	case *GeneralEmbeddingRequest:
+		return v.Copy()
+	default:
+		return nil
+	}
+}