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

.gitignore

@@ -0,0 +1,31 @@
+# Python-generated files
+__pycache__/
+*.py[oc]
+build/
+dist/
+wheels/
+*.egg-info
+
+# Virtual environments
+.venv
+
+# Database & logs
+*.db
+*.db-journal
+*.db-wal
+*.db-shm
+server.log
+nohup.out
+
+# Uploaded/generated assets
+uploads/
+thumbnails/
+images/
+
+# Test artifacts
+*.stl
+test.db
+
+# IDE
+.vscode/
+.idea/