chore: consolidate env files and update gitignore

Removed .env and .env.backup from git tracking and consolidated configuration into .env.example. Updated .gitignore to robustly prevent accidental inclusion of sensitive files.

.env.example

@@ -1,28 +1,43 @@
-# LLM Proxy Gateway Environment Variables
-# Copy to .env and fill in your API keys
+# LLM Proxy Gateway Configuration Example
+# Copy this file to .env and fill in your values
 
-# 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
+# ==============================================================================
+# MANDATORY: Encryption & Security
+# ==============================================================================
+# A 32-byte hex or base64 encoded string used for session signing and 
+# database encryption.
+# Generate one with: openssl rand -hex 32
 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
-GEMINI_API_KEY=your_gemini_api_key_here
-DEEPSEEK_API_KEY=your_deepseek_api_key_here
-GROK_API_KEY=your_grok_api_key_here
+# ==============================================================================
+# LLM Provider API Keys
+# ==============================================================================
+OPENAI_API_KEY=sk-...
+GEMINI_API_KEY=AIza...
+DEEPSEEK_API_KEY=sk-...
+GROK_API_KEY=xai-...
 
+# ==============================================================================
+# Server Configuration
+# ==============================================================================
+LLM_PROXY__SERVER__PORT=8080
+LLM_PROXY__SERVER__HOST=0.0.0.0
+
+# Optional: Bearer tokens for client authentication (comma-separated)
+# If not set, the proxy will look up tokens in the database.
+# LLM_PROXY__SERVER__AUTH_TOKENS=token1,token2
+
+# ==============================================================================
+# Database Configuration
+# ==============================================================================
+LLM_PROXY__DATABASE__PATH=./data/llm_proxy.db
+LLM_PROXY__DATABASE__MAX_CONNECTIONS=10
+
+# ==============================================================================
 # 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
-
-# Server Configuration
-LLM_PROXY__SERVER__PORT=8080
-LLM_PROXY__SERVER__HOST=0.0.0.0
-
-# Database Configuration
-LLM_PROXY__DATABASE__PATH=./data/llm_proxy.db
-LLM_PROXY__DATABASE__MAX_CONNECTIONS=10