feat: add variation spawner and content repurposing

Variation spawner: select an image asset → spawn N variations that keep the
same emotional pattern/visual style but explore different hook angles. Runs
a focused ad-creative-designer agent with the original as a Gemini reference
image. New assets link back via parentAssetId.

Content repurposing: resize an image to all other platform dimensions using
Sharp (cover crop). Captions are re-toned for the target platform via Claude
CLI. No external APIs needed — fully local.

- Add parentAssetId self-relation to Asset model
- lib/repurpose.ts: Sharp resize, platform format mapping, caption re-toning
- lib/variations.ts: asset DNA extraction, variation prompt builder, mini-pipeline
- API routes: /api/assets/[id]/repurpose (GET formats, POST resize)
- API routes: /api/assets/[id]/variations (GET existing, POST spawn)
- Repurpose modal: checkbox list of target formats
- Variation modal: count picker, async launch
- Asset card: Repurpose + Variations buttons on image assets
- Asset lineage: "Derived from" shown on child assets

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/api/assets/route.ts

@@ -35,7 +35,10 @@ export async function GET(request: Request) {
   const assets = await prisma.asset.findMany({
     where,
     orderBy: { createdAt: "desc" },
-    include: { campaign: { select: { name: true } } },
+    include: {
+      campaign: { select: { name: true } },
+      parentAsset: { select: { id: true, fileName: true } },
+    },
   });
 
   return Response.json(assets);