Add vendor, model and netmiko_device_type fields to templates

Add three new optional fields to the template schema:
- vendor: Device vendor (e.g., Cisco, Juniper, Huawei)
- model: Device model (e.g., ISR4451-X, MX204, NE40E)
- netmiko_device_type: Netmiko device type for automation (e.g., cisco_ios, juniper_junos, huawei)

These fields enable AI/LLM integration to determine the appropriate
Netmiko device type when automating network device configuration.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

gns3server/db/models/templates.py

@@ -36,6 +36,9 @@ class Template(BaseTable):
     builtin = Column(Boolean, default=False)
     usage = Column(String)
     template_type = Column(String)
+    vendor = Column(String)
+    model = Column(String)
+    netmiko_device_type = Column(String)
     compute_id = Column(String)
     images = relationship("Image", secondary=image_template_map, back_populates="templates")