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

app/database.py

@@ -0,0 +1,20 @@
+from sqlalchemy import create_engine
+from sqlalchemy.orm import sessionmaker, declarative_base
+import os
+
+DATABASE_URL = "sqlite:///./stl_repo.db"
+
+engine = create_engine(
+    DATABASE_URL, connect_args={"check_same_thread": False}
+)
+SessionLocal = sessionmaker(autocommit=False, autoflush=False, bind=engine)
+
+Base = declarative_base()
+
+
+def get_db():
+    db = SessionLocal()
+    try:
+        yield db
+    finally:
+        db.close()