feat: implementar 12 mejoras, tests, docs y optimizaciones

- Fase A: license templates, search history, cost estimator
- Fase B: import URL, bulk ZIP, batch download
- Fase C: comparison mode, mesh validation, measurement tool
- Fase D: cross-section clipping, overhang heatmap, layer animation
- Refactor Pydantic/SQLAlchemy warnings
- 24 tests pytest
- README actualizado
- WebP thumbnails, lazy loading, cache headers

main.py

@@ -0,0 +1,6 @@
+def main():
+    print("Hello from stl-repo!")
+
+
+if __name__ == "__main__":
+    main()