feat: add PostgreSQL database adapter support

AGENTS.md

@@ -85,6 +85,29 @@ If users are redirected incorrectly or permissions aren't working:
 3. Run `npx prisma generate`
 4. Update affected API routes and components
 
+### Database Provider Switching
+The application supports both SQLite (default) and PostgreSQL databases.
+
+**To switch between databases:**
+```bash
+# Switch to SQLite
+npm run db:switch sqlite
+
+# Switch to PostgreSQL  
+npm run db:switch postgresql
+```
+
+**To set up PostgreSQL:**
+```bash
+npm run db:setup-postgres
+```
+
+**Database Configuration:**
+- Edit `.env` file to set `DATABASE_PROVIDER` and `DATABASE_URL`
+- For PostgreSQL, also set `DATABASE_SHADOW_URL` for migrations
+
+**Note:** The database provider is automatically detected by Better Auth and Prisma.
+
 ### Running Tests
 - **Unit tests**: `npm run test`
 - **Acceptance tests**: `npm run test:acceptance`