Phase 25: templates as stack folders (0.31.0)

Templates are now stack-shaped folders (compose.yaml + .env.example +
template.json) instead of DB rows + manifest.json + {{VAR}} rendering.
Pull copies the folder into a new stack; custom templates persist under
DATA_DIR/templates. Adds POST /api/templates/from-stack and a one-time
startup migration for pre-0.31 DB templates (drops the template table).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/models/__init__.py

@@ -6,10 +6,9 @@ from models.backup_destination import BackupDestination
 from models.backup_schedule import BackupSchedule
 from models.setting import Setting, Webhook
 from models.stack import Stack
-from models.template import Template
 from models.user import User
 
 __all__ = [
-    "User", "Stack", "AuditLog", "Template", "Setting", "Webhook", "Agent",
+    "User", "Stack", "AuditLog", "Setting", "Webhook", "Agent",
     "BackupDestination", "BackupSchedule", "AutoUpdate",
 ]