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/gitea/.env.example

@@ -0,0 +1,5 @@
+PUID=1000
+PGID=1000
+DATA_PATH=/srv/gitea
+HTTP_PORT=3000
+SSH_PORT=2222

backend/templates/gitea/compose.yaml

@@ -0,0 +1,15 @@
+services:
+  gitea:
+    image: gitea/gitea:latest
+    container_name: gitea
+    restart: unless-stopped
+    environment:
+      - USER_UID=${PUID:-1000}
+      - USER_GID=${PGID:-1000}
+    ports:
+      - "${HTTP_PORT:-3000}:3000"
+      - "${SSH_PORT:-2222}:22"
+    volumes:
+      - ${DATA_PATH:-/srv/gitea}:/data
+      - /etc/timezone:/etc/timezone:ro
+      - /etc/localtime:/etc/localtime:ro

backend/templates/gitea/template.json

@@ -0,0 +1,9 @@
+{
+  "name": "Gitea",
+  "description": "Lightweight self-hosted Git service.",
+  "tags": [
+    "git",
+    "dev"
+  ],
+  "gpu": null
+}