feat: add network_diagrams table

Create NetworkDiagram SQLAlchemy model with JSONB nodes/edges, team-scoped with client/asset metadata, and Alembic migration 074.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/models/__init__.py

@@ -57,6 +57,7 @@ from .session_resolution_output import SessionResolutionOutput
 from .template_tree import TemplateTree
 from .platform_step import PlatformStep
 from .device_type import DeviceType
+from .network_diagram import NetworkDiagram
 
 __all__ = [
     "User",
@@ -128,4 +129,5 @@ __all__ = [
     "TemplateTree",
     "PlatformStep",
     "DeviceType",
+    "NetworkDiagram",
 ]