feat: add CORS middleware and SSE streaming endpoint

Add CORS support for frontend development with configurable origins via
CORS_ORIGINS environment variable. Add /chat/stream endpoint for
Server-Sent Events streaming with true streaming support for OpenAI
adapter and fallback single-chunk behavior for other adapters.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.env.example

@@ -1,6 +1,9 @@
 # LLM Mode: "local", "remote", "openai", or "asksage"
 LLM_MODE=local
 
+# CORS Configuration (comma-separated origins for frontend access)
+CORS_ORIGINS=http://localhost:3000
+
 # Remote LLM Configuration (required if LLM_MODE=remote)
 LLM_REMOTE_URL=https://your-llm-service.com/generate
 LLM_REMOTE_TOKEN=