feat: add seed test user migration

Add Flyway migration V2 that inserts a pre-seeded test user for manual
testing. This avoids having to register a new account every time the
environment is reset.

- Email: test@bilhalsning.se
- Password: test1234
- Password hash: bcrypt ($2b$12$)

The migration uses a plain INSERT (no ON CONFLICT) since it runs on
fresh databases only. H2-compatible — no PostgreSQL-specific syntax.
To re-seed after deletion: docker compose down -v && docker compose up -d

backend/src/main/resources/db/migration/V2__seed_test_user.sql

@@ -0,0 +1,7 @@
+INSERT INTO users (id, email, password_hash, subscription)
+VALUES (
+    'a0eebc99-9c0b-4ef8-bb6d-6bb9bd380a11',
+    'test@bilhalsning.se',
+    '$2b$12$18UFRDPgHWuw5FYeu6X1ReisFjjuxs5XxDafi6.wZbsywoU7vUaLG',
+    'none'
+);