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 The original Rust code has been archived in the 'rust' branch.

.env.example

@@ -1,31 +1,28 @@
 # LLM Proxy Gateway Environment Variables
 # Copy to .env and fill in your API keys
 
-# OpenAI
+# MANDATORY: Encryption key for sessions and stored API keys
+# Must be a 32-byte hex or base64 encoded string
+# Example (hex): LLM_PROXY__ENCRYPTION_KEY=0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef
+LLM_PROXY__ENCRYPTION_KEY=your_secure_32_byte_key_here
+
+# LLM Provider API Keys (Standard Environment Variables)
 OPENAI_API_KEY=your_openai_api_key_here
-
-# Google Gemini
 GEMINI_API_KEY=your_gemini_api_key_here
-
-# DeepSeek
 DEEPSEEK_API_KEY=your_deepseek_api_key_here
-
-# xAI Grok (not yet available)
 GROK_API_KEY=your_grok_api_key_here
 
-# Ollama (local server)
-# LLM_PROXY__PROVIDERS__OLLAMA__BASE_URL=http://your-ollama-host:11434/v1
+# Provider Overrides (Optional)
+# LLM_PROXY__PROVIDERS__OPENAI__BASE_URL=https://api.openai.com/v1
+# LLM_PROXY__PROVIDERS__GEMINI__ENABLED=true
+# LLM_PROXY__PROVIDERS__OLLAMA__BASE_URL=http://localhost:11434/v1
 # LLM_PROXY__PROVIDERS__OLLAMA__ENABLED=true
 # LLM_PROXY__PROVIDERS__OLLAMA__MODELS=llama3,mistral,llava
 
-# Authentication tokens (comma-separated list)
-LLM_PROXY__SERVER__AUTH_TOKENS=your_bearer_token_here,another_token
-
-# Server port (optional)
+# Server Configuration
 LLM_PROXY__SERVER__PORT=8080
+LLM_PROXY__SERVER__HOST=0.0.0.0
 
-# Database path (optional)
+# Database Configuration
 LLM_PROXY__DATABASE__PATH=./data/llm_proxy.db
-
-# Session secret for HMAC-signed tokens (hex or base64 encoded, 32 bytes)
-SESSION_SECRET=your_session_secret_here_32_bytes
+LLM_PROXY__DATABASE__MAX_CONNECTIONS=10