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/main.py

@@ -35,7 +35,7 @@ from routers import (
     volumes,
     ws,
 )
-from services import schedule_service, update_service
+from services import schedule_service, template_service, update_service
 
 logging.basicConfig(level=logging.INFO)
 logger = logging.getLogger("stackpilot")
@@ -50,6 +50,12 @@ async def lifespan(app: FastAPI):
             stacks.sync_discovered_stacks(session)
     except Exception as exc:  # noqa: BLE001
         logger.warning("Stack discovery failed: %s", exc)
+    try:
+        moved = template_service.migrate_legacy_db_templates()
+        if moved:
+            logger.info("Migrated %d custom template(s) from the database to folders", moved)
+    except Exception as exc:  # noqa: BLE001
+        logger.warning("Legacy template migration failed: %s", exc)
     update_task = asyncio.create_task(update_service.background_loop())
     schedule_task = asyncio.create_task(schedule_service.scheduler_loop())
     logger.info("StackPilot backend ready on port %s", settings.PORT)
@@ -58,7 +64,7 @@ async def lifespan(app: FastAPI):
     schedule_task.cancel()
 
 
-app = FastAPI(title="StackPilot", version="0.30.0", lifespan=lifespan)
+app = FastAPI(title="StackPilot", version="0.31.0", lifespan=lifespan)
 
 app.add_middleware(
     CORSMiddleware,