feat(network): thumbnail generation on save, shown on list page

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/schemas/network_diagram.py

@@ -92,6 +92,7 @@ class NetworkDiagramListItem(BaseModel):
     description: str | None = None
     node_count: int = 0
     category_counts: dict[str, int] = Field(default_factory=dict)
+    thumbnail_url: str | None = None
     created_by: UUID | None = None
     created_at: datetime
     updated_at: datetime