fix(ci): update references from db/schema.sql to sqlx migrations

CI and Docker builds broke after removing db/ in favor of migrations/.
- ci.yml: point psql init at migrations/20260307000000_initial_schema.sql
- Dockerfile: COPY migrations instead of db; remove unused db copy in final stage
- docker-build.yml: remove db/schema.sql presence check

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

.github/workflows/docker-build.yml

@@ -70,7 +70,4 @@ jobs:
           echo "=== Static files present ==="
           docker run --rm --entrypoint sh test/oxicloud:test -c "ls /app/static/index.html && echo 'OK: static files present'"
 
-          echo "=== DB schema present ==="
-          docker run --rm --entrypoint sh test/oxicloud:test -c "ls /app/db/schema.sql && echo 'OK: schema present'"
-
           echo "✅ Docker build and test completed successfully"