feat: web login system, auto-load history, config extraction

- Add Flask session-based authentication (username/password login)
- Auto-load chat history from backend on page load
- Extract SECRET_KEY and user credentials to web_config.json
- Create web_config.example.json as template
- Add web_config.json to .gitignore (sensitive info)
- Update docs: architecture, API reference, quick start guides

web_config.example.json

@@ -0,0 +1,6 @@
+{
+  "SECRET_KEY": "change-this-to-a-random-secret-key",
+  "USERS": {
+    "admin": "SHA256_OF_YOUR_PASSWORD"
+  }
+}