feat(backend): add user profile fields and profile/password endpoints

Add firstName, lastName, birthDate, contact to AppUser via V7 migration.
Add PUT /api/users/me and POST /api/users/me/password endpoints.
Add GET /api/users/{id} for public profile lookup.
Add EMAIL_ALREADY_IN_USE and WRONG_CURRENT_PASSWORD error codes.

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

backend/src/main/resources/db/migration/V7__add_profile_fields.sql

@@ -0,0 +1,5 @@
+ALTER TABLE users ADD COLUMN first_name VARCHAR(100);
+ALTER TABLE users ADD COLUMN last_name  VARCHAR(100);
+ALTER TABLE users ADD COLUMN birth_date DATE;
+ALTER TABLE users ADD COLUMN contact    TEXT;
+ALTER TABLE users ADD CONSTRAINT users_email_unique UNIQUE (email);