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

@@ -55,6 +55,7 @@ from .fork_point import ForkPoint
 from .session_handoff import SessionHandoff
 from .session_resolution_output import SessionResolutionOutput
 from .device_type import DeviceType
+from .network_diagram import NetworkDiagram
 
 __all__ = [
     "User",
@@ -124,4 +125,5 @@ __all__ = [
     "SessionHandoff",
     "SessionResolutionOutput",
     "DeviceType",
+    "NetworkDiagram",
 ]