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/vaultwarden/.env.example

@@ -0,0 +1,4 @@
+TZ=Europe/Berlin
+DATA_PATH=/srv/vaultwarden
+HTTP_PORT=8200
+ADMIN_TOKEN=change-me

backend/templates/vaultwarden/compose.yaml

@@ -0,0 +1,13 @@
+services:
+  vaultwarden:
+    image: vaultwarden/server:latest
+    container_name: vaultwarden
+    restart: unless-stopped
+    environment:
+      - TZ=${TZ:-Europe/Berlin}
+      - ADMIN_TOKEN=${ADMIN_TOKEN:-change-me}
+      - WEBSOCKET_ENABLED=true
+    ports:
+      - "${HTTP_PORT:-8200}:80"
+    volumes:
+      - ${DATA_PATH:-/srv/vaultwarden}:/data

backend/templates/vaultwarden/template.json

@@ -0,0 +1,9 @@
+{
+  "name": "Vaultwarden",
+  "description": "Lightweight Bitwarden-compatible password manager.",
+  "tags": [
+    "password-manager",
+    "security"
+  ],
+  "gpu": null
+}