feat(auth): add user profile centre and admin role system

- add GET/PUT /user/me endpoints for profile viewing and editing
- add PUT /user/me/password endpoint for password changes
- add GET /user/me/builds skeleton endpoint for future CustomModification
- add user_role table with ROLE_USER, ROLE_ADMIN, and ROLE_SUPER_USER roles
- add @RequiresRole annotation with custom AuthorizationManager enforcement
- extend User entity with nickname, avatarUrl, emailVerified, enabled,
  createdAt, and updatedAt columns
- load user roles into SecurityContext authorities via TokenAuthenticationFilter
- add @CurrentUser annotation and argument resolver for controller injection
- add PUT/DELETE /admin/users/{id}/roles for SUPER_USER role management
- add V6 and V7 Flyway migrations for schema changes

Closes #5
Closes #8

src/main/resources/db/migration/V7__user_role.sql

@@ -0,0 +1,6 @@
+CREATE TABLE user_role
+(
+    user_id BIGINT      NOT NULL REFERENCES app_user (id),
+    role    VARCHAR(32) NOT NULL,
+    CONSTRAINT user_role_pkey PRIMARY KEY (user_id, role)
+);