Implement auth domain with outside-in TDD (22 tests)

Controller (7 tests): signup, login, logout, GET/PATCH me.
Standalone MockMvc setup (Boot 4 removed @WebMvcTest).

Service (11 tests): signup with conflict check, login with
password/active validation, getCurrentUser with household info,
updateProfile with password change flow.

Repository (4 tests): save/find, case-insensitive email via
IgnoreCase queries (citext + Hibernate needs explicit IgnoreCase),
existsByEmail.

Also includes:
- SecurityConfig: session auth, CSRF, role-based authorization
- CustomUserDetailsService: loads UserAccount for Spring Security
- UserAccount, Household, HouseholdMember JPA entities
- spring-boot-flyway dependency (Boot 4 requires explicit module)
- ddl-auto=none (Flyway owns schema, validate fails on citext)

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

backend/src/main/java/com/recipeapp/household/HouseholdMemberRepository.java

@@ -0,0 +1,11 @@
+package com.recipeapp.household;
+
+import com.recipeapp.household.entity.HouseholdMember;
+import org.springframework.data.jpa.repository.JpaRepository;
+
+import java.util.Optional;
+import java.util.UUID;
+
+public interface HouseholdMemberRepository extends JpaRepository<HouseholdMember, UUID> {
+    Optional<HouseholdMember> findByUserEmailIgnoreCase(String email);
+}