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/paperless-ngx/compose.yaml

@@ -0,0 +1,38 @@
+services:
+  broker:
+    image: redis:7-alpine
+    container_name: paperless-redis
+    restart: unless-stopped
+    volumes:
+      - ${DATA_PATH:-/srv/paperless}/redis:/data
+
+  db:
+    image: postgres:16-alpine
+    container_name: paperless-db
+    restart: unless-stopped
+    environment:
+      - POSTGRES_DB=paperless
+      - POSTGRES_USER=paperless
+      - POSTGRES_PASSWORD=paperless
+    volumes:
+      - ${DATA_PATH:-/srv/paperless}/db:/var/lib/postgresql/data
+
+  webserver:
+    image: ghcr.io/paperless-ngx/paperless-ngx:latest
+    container_name: paperless
+    restart: unless-stopped
+    depends_on:
+      - db
+      - broker
+    environment:
+      - PAPERLESS_REDIS=redis://broker:6379
+      - PAPERLESS_DBHOST=db
+      - PAPERLESS_TIME_ZONE=${TZ:-Europe/Berlin}
+      - PAPERLESS_ADMIN_USER=${ADMIN_USER:-admin}
+      - PAPERLESS_ADMIN_PASSWORD=${ADMIN_PASSWORD:-change-me}
+    ports:
+      - "${HTTP_PORT:-8000}:8000"
+    volumes:
+      - ${DATA_PATH:-/srv/paperless}/data:/usr/src/paperless/data
+      - ${DATA_PATH:-/srv/paperless}/media:/usr/src/paperless/media
+      - ${DATA_PATH:-/srv/paperless}/consume:/usr/src/paperless/consume