Add backend-only OIDC login documentation

docs/development/backend/oidc.md

@@ -0,0 +1,49 @@
+This backend-only implementation adds OpenID Connect (OIDC) login via Google, GitHub, and Microsoft using [Authlib](https://docs.authlib.org).
+
+## Features
+Role-based login with Google, GitHub, Microsoft
+Backend-only logic (no frontend UI changes)
+Session-based user info (not persisted in DB)
+
+## Setup Instructions
+
+1. Install dependencies
+
+```bash
+pip install Authlib python-dotenv
+```
+
+1. Copy and fill your .env file
+
+```bash
+cp .env.example .env
+```
+3. Update .env with your OIDC client IDs and secrets
+
+4. Run the app
+
+```bash
+flask run
+```
+5. Login
+
+Visit:
+http://localhost:5000/auth/login/google
+
+
+http://localhost:5000/auth/login/github
+
+
+http://localhost:5000/auth/login/microsoft
+
+
+## File Structure
+
+.env.example
+auth/
+└── oidc.py          # OIDC logic
+gramps_webapi/
+└── app.py           # Registers blueprint and OAuth setup
+
+## Test
+After login, you’ll be redirected back and shown user info. Session storage is temporary and can be extended for persistence.