docs: Update GEMINI.md with backend API endpoints and frontend architecture details

GEMINI.md

@@ -8,8 +8,39 @@ This project is an Election Analytics Chatbot built with a modern, stateful, and
 - **[Frontend](./frontend/GEMINI.md)**: React application for user interaction.
 
 ## Key Technologies
-- **Backend**: LangGraph, LangChain, OpenAI/Google Gemini, PostgreSQL.
-- **Frontend**: React, TypeScript, Vite.
+- **Backend**: LangGraph, LangChain, OpenAI/Google Gemini, PostgreSQL, FastAPI.
+- **Frontend**: React, TypeScript, Vite, Tailwind CSS, Shadcn UI.
+
+## Backend API (`/api/v1`)
+The backend provides a versioned REST and Streaming API:
+
+### Authentication (`/auth`)
+- `POST /register`: New user registration.
+- `POST /login`: Email/password login setting an `access_token` HttpOnly cookie.
+- `POST /logout`: Clears the authentication cookie.
+- `GET /me`: Returns the current authenticated user's profile.
+- `GET /oidc/login` & `/oidc/callback`: Full OIDC/SSO integration.
+
+### Agent execution (`/chat`)
+- `POST /stream`: A stateful SSE (Server-Sent Events) endpoint that streams the LangGraph agent's reasoning steps, model chunks, and generated plots in real-time.
+
+### History Management (`/conversations`)
+- `GET /`: List all conversations for the current user.
+- `POST /`: Create a new conversation.
+- `GET /{id}/messages`: Retrieve full message history including step-by-step reasoning.
+- `PATCH /{id}`: Rename or update conversation summaries.
+- `DELETE /{id}`: Persistent deletion of chat history.
+
+### Artifacts (`/artifacts`)
+- `GET /plots/{plot_id}`: Secure binary retrieval of generated analysis charts (PNG).
+
+## Frontend Architecture
+The frontend is a modern SPA (Single Page Application) designed for data-heavy interactions:
+
+- **State Management**: React-based state with Axios interceptors for automatic session handling (401 redirects).
+- **Authentication**: Seamlessly handles both traditional and OIDC login flows, persisting sessions via browser-native cookie handling.
+- **UI System**: Built with Shadcn UI and Tailwind CSS, featuring a responsive sidebar-based layout for conversation management.
+- **Real-time Visualization**: Supports streaming text responses and immediate rendering of base64-encoded or binary-retrieved analysis plots.
 
 ## Documentation
 - **[Backend Guide](./backend/GEMINI.md)**: Detailed information about the backend architecture, migration goals, and implementation steps.