feat: migrate backend from rust to go

This commit replaces the Axum/Rust backend with a Gin/Go implementation. The original Rust code has been archived in the 'rust' branch.

TODO.md

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+# Migration TODO List
+
+## Completed Tasks
+- [x] Initial Go project setup
+- [x] Database schema & migrations
+- [x] Configuration loader (Viper)
+- [x] Auth Middleware
+- [x] Basic Provider implementations (OpenAI, Gemini, DeepSeek, Grok)
+- [x] Streaming Support (SSE & Gemini custom streaming)
+- [x] Move Rust files to `rust_backup`
+- [x] Enhanced `helpers.go` for Multimodal & Tool Calling (OpenAI compatible)
+- [x] Enhanced `server.go` for robust request conversion
+- [x] Dashboard Management APIs (Clients, Tokens, Users, Providers)
+- [x] Dashboard Analytics & Usage Summary
+- [x] WebSocket for real-time dashboard updates
+
+## Feature Parity Checklist (High Priority)
+
+### OpenAI Provider
+- [x] Tool Calling
+- [x] Multimodal (Images) support
+- [ ] Reasoning Content (CoT) support for `o1`, `o3` (need to ensure it's parsed in responses)
+- [ ] Support for `/v1/responses` API (required for some gpt-5/o1 models)
+
+### Gemini Provider
+- [x] Tool Calling (mapping to Gemini format)
+- [x] Multimodal (Images) support
+- [x] Reasoning/Thought support
+- [x] Handle Tool Response role in unified format
+
+### DeepSeek Provider
+- [x] Reasoning Content (CoT) support
+- [x] Parameter sanitization for `deepseek-reasoner`
+- [x] Tool Calling support
+
+### Grok Provider
+- [x] Tool Calling support
+- [x] Multimodal support
+
+## Infrastructure & Middleware
+- [ ] Implement Request Logging to SQLite (asynchronous)
+- [ ] Implement Rate Limiting (`golang.org/x/time/rate`)
+- [ ] Implement Circuit Breaker (`github.com/sony/gobreaker`)
+- [ ] Implement Model Cost Calculation logic
+
+## Verification
+- [ ] Unit tests for feature-specific mapping (CoT, Tools, Images)
+- [ ] Integration tests with live LLM APIs