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/jellyfin/template.json

@@ -0,0 +1,9 @@
+{
+  "name": "Jellyfin",
+  "description": "Free media streaming server with optional hardware transcoding.",
+  "tags": [
+    "media",
+    "streaming"
+  ],
+  "gpu": "NVIDIA / Intel optional"
+}