Fix dependency graph error for invalid version constraints

When a dependency has an invalid version constraint like '>=' (without
a version number), the resolver now treats it as a wildcard and returns
the latest available version instead of failing with 'Dependency not found'.

This handles malformed metadata that may have been stored from PyPI packages.

backend/app/dependencies.py

@@ -358,7 +358,15 @@ def _resolve_version_constraint(
     try:
         specifier = SpecifierSet(constraint)
     except InvalidSpecifier:
-        # Invalid constraint, try as exact version
+        # Invalid constraint (e.g., ">=" without version) - treat as wildcard
+        # This can happen with malformed metadata from PyPI packages
+        latest = db.query(PackageVersion).filter(
+            PackageVersion.package_id == package.id,
+        ).order_by(PackageVersion.created_at.desc()).first()
+        if latest:
+            artifact = db.query(Artifact).filter(Artifact.id == latest.artifact_id).first()
+            if artifact:
+                return (artifact.id, latest.version, artifact.size)
         return None
 
     # Get all versions for this package