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>

prisma/schema.prisma

@@ -83,6 +83,9 @@ model Asset {
   publishedTo   String? // JSON array of platforms published to
   postizPostId  String?
   postizMediaId String?
+  parentAssetId String?
+  parentAsset   Asset?    @relation("AssetVariations", fields: [parentAssetId], references: [id])
+  variations    Asset[]   @relation("AssetVariations")
   createdAt     DateTime  @default(now())
 }