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/test/resources/application-test.yml

@@ -1,12 +1,8 @@
 spring:
-  datasource:
-    url: jdbc:tc:postgresql:16-alpine:///mealprep
-    username: mealprep
-    password: mealprep
   jpa:
     open-in-view: false
     hibernate:
-      ddl-auto: validate
+      ddl-auto: none
   flyway:
     enabled: true
     locations: classpath:db/migration