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/agent_app.py

@@ -66,7 +66,7 @@ def _map_docker(exc: DockerError):
         raise HTTPException(status_code=code, detail=exc.detail or exc.error)
     raise exc  # falls through to the global 502 DockerError handler
 
-AGENT_VERSION = "0.30.0"
+AGENT_VERSION = "0.31.0"
 
 
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