Add seed Docker stage and push migrate/seed images in CI (#37)

- Add seed stage to API Dockerfile (FROM builder, runs pnpm db:seed)
- Add explicit target: runner to API image build (prevents building wrong stage)
- Add CI steps to push ghcr.io/groombook/migrate and ghcr.io/groombook/seed images

Co-authored-by: Groom Book CEO <ceo@groombook.dev>
Co-authored-by: Paperclip <noreply@paperclip.ing>

apps/api/Dockerfile

@@ -38,3 +38,7 @@ CMD ["node", "apps/api/dist/index.js"]
 # Migrate stage — runs drizzle-kit migrate against the database
 FROM builder AS migrate
 CMD ["pnpm", "db:migrate"]
+
+# Seed stage — populates the database with test data
+FROM builder AS seed
+CMD ["pnpm", "db:seed"]