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/src/main/resources/db/migration/V8__add_password_reset_tokens.sql

@@ -0,0 +1,10 @@
+CREATE TABLE password_reset_tokens (
+    id          UUID         PRIMARY KEY DEFAULT gen_random_uuid(),
+    user_id     UUID         NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    token       VARCHAR(64)  NOT NULL UNIQUE,
+    expires_at  TIMESTAMP    NOT NULL,
+    used        BOOLEAN      NOT NULL DEFAULT FALSE,
+    created_at  TIMESTAMP    NOT NULL DEFAULT now()
+);
+
+CREATE INDEX idx_prt_token ON password_reset_tokens(token);