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/templates/uptime-kuma/compose.yaml

@@ -0,0 +1,9 @@
+services:
+  uptime-kuma:
+    image: louislam/uptime-kuma:1
+    container_name: uptime-kuma
+    restart: unless-stopped
+    ports:
+      - "${HTTP_PORT:-3001}:3001"
+    volumes:
+      - ${DATA_PATH:-/srv/uptime-kuma}:/app/data