templates: add netmiko_device_type for automation tools

Common template field (schema + templates table column + Alembic
migration) holding the Netmiko device type (e.g. 'cisco_xr', 'nokia_srl')
so Netmiko/Nornir based tooling can look up how to reach a node's CLI
without hard-coded vendor mappings. Free-form lowercase string on
purpose: Netmiko's platform list evolves independently of GNS3.

gns3server/db_migrations/versions/b3c7e2a91d4f_add_netmiko_device_type_to_templates.py

@@ -0,0 +1,26 @@
+"""add netmiko_device_type to templates table
+
+Revision ID: b3c7e2a91d4f
+Revises: 8f2a1c4e9d3b
+Create Date: 2026-08-16 10:00:00.000000
+
+"""
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision = 'b3c7e2a91d4f'
+down_revision = '8f2a1c4e9d3b'
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+
+    op.add_column('templates', sa.Column('netmiko_device_type', sa.String()))
+
+
+def downgrade() -> None:
+
+    op.drop_column('templates', 'netmiko_device_type')