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/models.py

@@ -64,6 +64,7 @@ class Post(Base):
     __tablename__ = "posts"
 
     id = Column(Integer, primary_key=True, index=True)
+    batch_id = Column(String(36), nullable=True, index=True)  # groups posts generated together
     character_id = Column(Integer, ForeignKey("characters.id"), nullable=False)
     content_type = Column(String(20), default="text")  # text, image, video, carousel
     text_content = Column(Text)