feat(zerotier): add ZeroTier network governance module

Add comprehensive ZeroTier integration for managing network access:

- Portal networks: manager-created ZeroTier network bindings
- Device registration: user-owned ZeroTier node endpoints
- Approval workflows: request/approve/revoke network access
- Activation sessions: time-limited network authorization
- Kill switch: emergency access revocation
- Reconciliation job: sync portal state with ZeroTier controller

Includes ZeroTier client SDK supporting both Central and self-hosted
controller APIs, with full CRUD operations for networks and members.

gatehouse_app/exceptions/__init__.py

@@ -34,6 +34,19 @@ from gatehouse_app.exceptions.ssh_exceptions import (
     DepartmentError,
     DepartmentNotFoundError,
 )
+from gatehouse_app.exceptions.zerotier_exceptions import (
+    ZeroTierAPIError,
+    ZeroTierAuthError,
+    ZeroTierNotFoundError,
+    NetworkNotFoundError,
+    DeviceNotFoundError,
+    ApprovalNotFoundError,
+    MembershipNotFoundError,
+    DeviceAlreadyExistsError,
+    ApprovalAlreadyExistsError,
+    InvalidNodeIdError,
+    InvalidNetworkIdError,
+)
 
 __all__ = [
     "BaseAPIException",
@@ -65,5 +78,16 @@ __all__ = [
     "PrincipalNotFoundError",
     "DepartmentError",
     "DepartmentNotFoundError",
+    "ZeroTierAPIError",
+    "ZeroTierAuthError",
+    "ZeroTierNotFoundError",
+    "NetworkNotFoundError",
+    "DeviceNotFoundError",
+    "ApprovalNotFoundError",
+    "MembershipNotFoundError",
+    "DeviceAlreadyExistsError",
+    "ApprovalAlreadyExistsError",
+    "InvalidNodeIdError",
+    "InvalidNetworkIdError",
 ]