feat: implement user profile endpoints (task 31)

- Add GET /api/users/:id for profile retrieval with stats
- Add PATCH /api/users/:id for profile updates
- Add GET /api/users/:id/reports to fetch user's reports
- Add GET /api/users/:id/events to fetch user's events (registered & organized)
- Add PATCH /api/users/:id/preferences for notification settings
- Add POST /api/users/:id/calculate-impact for impact score calculation
- Create userService with all business logic
- Create userController with request handling
- Create userRoutes and register in main server
- Update frontend userService with new endpoints
- Add comprehensive documentation in USER_PROFILE_ENDPOINTS.md
- Add test script for manual endpoint testing

Requirements satisfied: 6.3, 6.4, 6.5, 10.4

.kiro/specs/beach-litter-mapping/tasks.md

@@ -117,7 +117,7 @@
 
 ## User Profile and Engagement
 
-- [x] 15. Build user profile page
+- [ ] 15. Build user profile page
   - Display user statistics (reports, events attended)
   - Calculate and show impact score
   - Create personal contribution map
@@ -259,7 +259,7 @@
   - Create verification audit log
   - _Requirements: 7.1, 7.2, 7.3, 7.4, 7.5_
 
-- [ ] 31. Build user profile endpoints
+- [-] 31. Build user profile endpoints
   - Create GET /api/users/:id for profile retrieval
   - Implement PATCH /api/users/:id for profile updates
   - Build GET /api/users/:id/reports endpoint