feat: change auto_fetch default to true

Auto-fetching missing dependencies from upstream is the more useful default
behavior. Users who need fast, network-free resolution can explicitly set
auto_fetch=false.

Artifacts are content-addressed by SHA256, so reproducibility concerns don't
apply - the same version always produces the same artifact.

backend/app/routes.py

@@ -7032,8 +7032,8 @@ async def resolve_artifact_dependencies(
     ref: str,
     request: Request,
     auto_fetch: bool = Query(
-        False,
-        description="Fetch missing dependencies from upstream registries (e.g., PyPI)"
+        True,
+        description="Fetch missing dependencies from upstream registries (e.g., PyPI). Set to false for fast, network-free resolution."
     ),
     db: Session = Depends(get_db),
     storage: S3Storage = Depends(get_storage),
@@ -7046,9 +7046,9 @@ async def resolve_artifact_dependencies(
     (dependencies before dependents). Includes download URLs for each artifact.
 
     **Parameters:**
-    - **auto_fetch**: When true, attempts to fetch missing dependencies from
-      upstream registries (PyPI for _pypi project packages). Default is false
-      for fast, network-free resolution.
+    - **auto_fetch**: When true (default), attempts to fetch missing dependencies from
+      upstream registries (PyPI for _pypi project packages). Set to false for
+      fast, network-free resolution when all dependencies are already cached.
 
     **Response Fields:**
     - **resolved**: All artifacts in dependency order with download URLs