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 The original Rust code has been archived in the 'rust' branch.

BACKEND_ARCHITECTURE.md

@@ -0,0 +1,61 @@
+# Backend Architecture (Go)
+
+The LLM Proxy backend is implemented in Go, focusing on high performance, clear concurrency patterns, and maintainability.
+
+## Core Technologies
+
+- **Runtime:** Go 1.22+
+- **Web Framework:** [Gin Gonic](https://github.com/gin-gonic/gin) - Fast and lightweight HTTP routing.
+- **Database:** [sqlx](https://github.com/jmoiron/sqlx) - Lightweight wrapper for standard `database/sql`.
+- **SQLite Driver:** [modernc.org/sqlite](https://modernc.org/sqlite) - CGO-free SQLite implementation for ease of cross-compilation.
+- **Config:** [Viper](https://github.com/spf13/viper) - Robust configuration management supporting environment variables and files.
+
+## Project Structure
+
+```text
+├── cmd/
+│   └── llm-proxy/          # Entry point (main.go)
+├── internal/
+│   ├── config/             # Configuration loading and validation
+│   ├── db/                 # Database schema, migrations, and models
+│   ├── middleware/         # Auth and logging middleware
+│   ├── models/             # Unified request/response structs
+│   ├── providers/          # LLM provider implementations (OpenAI, Gemini, etc.)
+│   ├── server/             # HTTP server, dashboard handlers, and WebSocket hub
+│   └── utils/              # Common utilities (multimodal, etc.)
+└── static/                 # Frontend assets (served by the backend)
+```
+
+## Key Components
+
+### 1. Provider Interface (`internal/providers/provider.go`)
+Standardized interface for all LLM backends:
+```go
+type Provider interface {
+	Name() string
+	ChatCompletion(ctx context.Context, req *models.UnifiedRequest) (*models.ChatCompletionResponse, error)
+	ChatCompletionStream(ctx context.Context, req *models.UnifiedRequest) (<-chan *models.ChatCompletionStreamResponse, error)
+}
+```
+
+### 2. Asynchronous Logging (`internal/server/logging.go`)
+Uses a buffered channel and background worker to log every request to SQLite without blocking the client response. It also broadcasts logs to the WebSocket hub for real-time dashboard updates.
+
+### 3. Session Management (`internal/server/sessions.go`)
+Implements HMAC-SHA256 signed tokens for dashboard authentication. Sessions are stored in-memory with configurable TTL.
+
+### 4. WebSocket Hub (`internal/server/websocket.go`)
+A centralized hub for managing WebSocket connections, allowing real-time broadcast of system events and request logs to the dashboard.
+
+## Concurrency Model
+
+Go's goroutines and channels are used extensively:
+- **Streaming:** Each streaming request uses a goroutine to read and parse the provider's response, feeding chunks into a channel.
+- **Logging:** A single background worker processes the `logChan` to perform database writes.
+- **WebSocket:** The `Hub` runs in a dedicated goroutine, handling registration and broadcasting.
+
+## Security
+
+- **Encryption Key:** A mandatory 32-byte key is used for both session signing and encryption of sensitive data in the database.
+- **Auth Middleware:** Verifies client API keys against the database before proxying requests to LLM providers.
+- **Bcrypt:** Passwords for dashboard users are hashed using Bcrypt with a work factor of 12.