feat: group posts by batch in archive + fullscreen confirm modal

Backend:
- Add batch_id column to Post model (UUID, groups posts from same generation)
- Set batch_id in /generate endpoint for all posts in same request

Frontend:
- ContentArchive: group posts by batch_id into single cards with platform tabs
- Character name at top, platform tabs below, status badge, text preview
- Click platform tab to switch between variants of same content
- ConfirmModal: render via React portal to document.body for true fullscreen overlay
- Add box-shadow and higher z-index for better visual separation

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

backend/app/schemas.py

@@ -82,6 +82,7 @@ class PostUpdate(BaseModel):
 
 class PostResponse(BaseModel):
     id: int
+    batch_id: Optional[str] = None
     character_id: int
     content_type: str
     text_content: Optional[str] = None