hotfix(checklists): disambiguate /checklists/{slug}/edit → /checklists/templates/{slug}/edit (production-down route conflict)

Go ServeMux refused to register patterns 'GET /checklists/{slug}/edit' (from
dirac's Slice A merge b418705) and 'GET /checklists/instances/{id}' (existing)
because both match '/checklists/instances/edit'. Container crash-looped on
boot since 13:32 UTC; paliad.de returned 404 from Traefik because no app was
listening.

Renaming the new template-edit route to /checklists/templates/{slug}/edit
disambiguates — '/templates/...' is a literal segment so the {slug} is now
strictly under a fixed prefix that can't collide with 'instances'.

Touches:
- internal/handlers/handlers.go:257 — route pattern
- frontend/src/client/checklists.ts:290 — Bearbeiten link
- frontend/src/client/checklists-author.ts:52 — URL parser regex
- frontend/src/checklists-author.tsx — doc comment

go build + bun run build clean.

frontend/src/client/checklists.ts

@@ -287,7 +287,7 @@ function renderMyTemplates() {
       <p class="checklist-card-desc">${esc(tpl.description || "")}</p>
       <p class="checklist-card-court">${esc(tpl.court || "")}</p>
       <div class="checklist-card-actions">
-        <a class="btn btn-small" href="/checklists/${esc(tpl.slug)}/edit" data-i18n="checklisten.mine.edit">Bearbeiten</a>
+        <a class="btn btn-small" href="/checklists/templates/${esc(tpl.slug)}/edit" data-i18n="checklisten.mine.edit">Bearbeiten</a>
         <button class="btn btn-small btn-danger" data-action="delete" data-slug="${esc(tpl.slug)}" data-title="${escAttr(tpl.title)}" data-i18n="checklisten.mine.delete">L&ouml;schen</button>
       </div>
     </article>`;