feat(studio): copy template scene graph into editable project (use-template works)

Build now created an EMPTY project: (1) the studio binds camelCase but the frontend
sent snake_case → original_project_id dropped to Guid.Empty; (2) CreateProjectAsync
never copied scenes. Now:
- saved-projects.ts sends camelCase (originalProjectId/copyDefaultValues).
- /api/projects resolves the container slug → first published variant content project.
- StudioService.CreateProjectAsync deep-copies the content scene graph (scenes +
  content elements + scene colours + shared colours) into the new saved project via
  one atomic cross-schema SQL copy (enum cols cast to text; temp scene-id map).

Verified: insta-promo → 1 scene, 6 content fields, 4 shared colours, loadable by the
studio editor.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/api/projects/route.ts

@@ -1,6 +1,7 @@
 import { NextResponse } from "next/server";
 import { z } from "zod";
 
+import { gatewayFetch } from "@/lib/api/gateway";
 import {
   createSavedProject,
   listSavedProjects,
@@ -10,6 +11,30 @@ import { getAccessToken } from "@/lib/auth/session";
 
 export const dynamic = "force-dynamic";
 
+const UUID_RE = /^[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}$/i;
+
+/**
+ * The studio copies from a content PROJECT (a specific aspect variant, UUID). Catalog
+ * links carry the container SLUG, so resolve slug → first published variant project id.
+ * A value that is already a UUID is returned unchanged.
+ */
+async function resolveContentProjectId(
+  idOrSlug: string,
+  token: string
+): Promise<string | null> {
+  if (UUID_RE.test(idOrSlug)) return idOrSlug;
+  const res = await gatewayFetch(`/v1/templates/${encodeURIComponent(idOrSlug)}`, {
+    headers: { Authorization: `Bearer ${token}` },
+  });
+  if (!res.ok) return null;
+  const tpl = (await res.json().catch(() => null)) as {
+    projects?: Array<{ id?: string; is_published?: boolean }>;
+  } | null;
+  const variants = tpl?.projects ?? [];
+  const chosen = variants.find((p) => p?.id && p.is_published) ?? variants.find((p) => p?.id);
+  return chosen?.id ?? null;
+}
+
 const createProjectSchema = z.object({
   name: z.string().min(1).max(120).optional(),
   type: z.enum(["video", "image", "trimmer"]),
@@ -71,8 +96,18 @@ export async function POST(request: Request) {
     );
   }
 
+  // Catalog links carry the container slug; the studio needs a content project UUID
+  // to copy the scene graph from.
+  const contentProjectId = await resolveContentProjectId(originalProjectId, token);
+  if (!contentProjectId) {
+    return NextResponse.json(
+      { error: "This template has no editable project yet.", code: "NO_VARIANT" },
+      { status: 422 }
+    );
+  }
+
   const result = await createSavedProject({
-    original_project_id: originalProjectId,
+    original_project_id: contentProjectId,
     name: parsed.data.name,
     copy_default_values: true,
   });

src/lib/api/saved-projects.ts

@@ -173,9 +173,16 @@ export async function createSavedProject(body: {
   preset_story_id?: string;
   copy_default_values?: boolean;
 }): Promise<StudioResult<V2SavedProjectFull>> {
+  // The studio service binds camelCase JSON (no snake_case naming policy), so a
+  // snake_case body silently drops original_project_id → Guid.Empty. Send camelCase.
   return studioFetch<V2SavedProjectFull>("/v1/saved-projects", {
     method: "POST",
-    body: JSON.stringify(body),
+    body: JSON.stringify({
+      originalProjectId: body.original_project_id,
+      name: body.name,
+      presetStoryId: body.preset_story_id,
+      copyDefaultValues: body.copy_default_values ?? true,
+    }),
   });
 }