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>

ROADMAP.md

@@ -208,6 +208,24 @@ RemoteStackDetail** (not the new-stack editor, which has no dir yet).
 
 ---
 
+## Phase 25 — Templates as stack folders   ☑ DONE — shipped 0.31.0
+
+Templates reworked from DB rows + `manifest.json` + `{{VAR}}` mustache rendering
+to **stack-shaped folders**: `backend/templates/<slug>/` with `compose.yaml`,
+optional `.env.example` and a `template.json` (name/description/tags/gpu).
+"Pull" copies the whole folder into a new stack (`.env.example` → `.env`);
+custom templates live under `${DATA_DIR}/templates/` and are written by
+"Save as template" (stack detail) or the manual save endpoint.
+
+- Backend: `template_service` rewritten (folder scan, traversal-guarded resolve,
+  `copy_into_stack`, `save_from_stack`); `Template` DB table dropped with a
+  one-time startup migration (`{{VAR}}` → `${VAR}`, vars → `.env.example`).
+- API: `POST /api/templates/from-stack` new; instantiate no longer takes `values`.
+- Frontend: Templates page shows compose/env preview + file list, delete for
+  custom templates; StackDetail gains "Save as template".
+
+---
+
 ## After 23
 Remaining un-built ideas from the gap analysis (not chosen this round):
 Health-monitoring & alerting (Docker-events → notify; note `/ws/events` already