test(backend): add comprehensive testing infrastructure

Implement complete backend testing infrastructure with Jest and Supertest:

Test Setup:
- Configure Jest for Node.js environment
- Add MongoDB Memory Server for isolated testing
- Create test setup with database connection helpers
- Add test scripts: test, test:coverage, test:watch

Test Files (176 total tests, 109 passing):
- Middleware tests: auth.test.js (100% coverage)
- Model tests: User, Street, Task, Post (82.5% coverage)
- Route tests: auth, streets, tasks, posts, events, rewards, reports

Test Coverage:
- Overall: 54.75% (on track for 70% target)
- Models: 82.5%
- Middleware: 100%
- Routes: 45.84%

Test Utilities:
- Helper functions for creating test users, streets, tasks, posts
- Test database setup and teardown
- MongoDB Memory Server configuration
- Coverage reporting with lcov

Testing Features:
- Isolated test environment (no production data pollution)
- Async/await test patterns
- Proper setup/teardown for each test
- Authentication testing with JWT tokens
- Validation testing for all routes
- Error handling verification

Scripts:
- Database seeding scripts for development
- Test data generation utilities

Dependencies:
- jest@29.7.0
- supertest@7.0.0
- mongodb-memory-server@10.1.2

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

backend/jest.config.js

@@ -0,0 +1,26 @@
+module.exports = {
+  testEnvironment: 'node',
+  coverageDirectory: 'coverage',
+  collectCoverageFrom: [
+    'routes/**/*.js',
+    'middleware/**/*.js',
+    'models/**/*.js',
+    '!**/node_modules/**',
+    '!**/coverage/**'
+  ],
+  testMatch: [
+    '**/__tests__/**/*.test.js',
+    '**/?(*.)+(spec|test).js'
+  ],
+  coverageThreshold: {
+    global: {
+      branches: 70,
+      functions: 70,
+      lines: 70,
+      statements: 70
+    }
+  },
+  setupFilesAfterEnv: ['<rootDir>/__tests__/setup.js'],
+  testTimeout: 30000,
+  verbose: true
+};