feat: add device_types table with system seed data

Creates DeviceType SQLAlchemy model and migration 073 that provisions the
device_types table with 28 system-seeded device types across 7 categories
(network, compute, storage, cloud, endpoint, infrastructure, security).

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

backend/app/models/__init__.py

@@ -54,6 +54,7 @@ from .session_branch import SessionBranch
 from .fork_point import ForkPoint
 from .session_handoff import SessionHandoff
 from .session_resolution_output import SessionResolutionOutput
+from .device_type import DeviceType
 
 __all__ = [
     "User",
@@ -122,4 +123,5 @@ __all__ = [
     "ForkPoint",
     "SessionHandoff",
     "SessionResolutionOutput",
+    "DeviceType",
 ]