feat(backend): add password reset via email

- Add PasswordResetToken entity, repository (Flyway V8 migration)
- PasswordResetService: token generation, validation, nightly cleanup
- AuthController: POST /api/auth/forgot-password and /api/auth/reset-password (both permitAll)
- AuthE2EController (@Profile("e2e")): GET /api/auth/reset-token-for-test for CI testing
- spring-boot-starter-mail dependency; JavaMailSender optional (@Autowired required=false)
- mail health indicator disabled; mail config via MAIL_HOST/PORT/USERNAME/PASSWORD env vars
- 5 unit tests written TDD-style (all pass)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/pom.xml

@@ -119,6 +119,10 @@
 			<artifactId>lombok</artifactId>
 			<optional>true</optional>
 		</dependency>
+		<dependency>
+			<groupId>org.springframework.boot</groupId>
+			<artifactId>spring-boot-starter-mail</artifactId>
+		</dependency>
 		<dependency>
 			<groupId>org.flywaydb</groupId>
 			<artifactId>flyway-core</artifactId>