feat(studio,stacking,animations,collections): M16 ModBison Studio, object stacking, model animations, and poly collections for MVP 2

backend/app/models/asset_collection.py

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+from sqlalchemy import Column, Integer, String, ForeignKey, Text, DateTime, Boolean
+from sqlalchemy.orm import relationship
+from sqlalchemy.sql import func
+
+from app.database import Base
+
+
+class AssetCollection(Base):
+    """A curated collection or pack of 3D models and assets."""
+    __tablename__ = "asset_collections"
+
+    id = Column(Integer, primary_key=True, index=True)
+    title = Column(String(255), nullable=False, index=True)
+    description = Column(Text, nullable=True)
+    theme = Column(String(64), nullable=False, default="general", index=True)
+    thumbnail_url = Column(String(512), nullable=True)
+    owner_id = Column(Integer, ForeignKey("users.id", ondelete="CASCADE"), nullable=False)
+    is_public = Column(Boolean, default=True, nullable=False, index=True)
+    tags = Column(String(512), nullable=True)
+    created_at = Column(DateTime(timezone=True), server_default=func.now())
+    updated_at = Column(DateTime(timezone=True), server_default=func.now(), onupdate=func.now())
+
+    owner = relationship("User", back_populates="asset_collections")
+    items = relationship(
+        "CollectionAsset",
+        back_populates="collection",
+        cascade="all, delete-orphan",
+        order_by="CollectionAsset.sort_order",
+    )
+
+
+class CollectionAsset(Base):
+    """Join entity linking an Asset to an AssetCollection with ordering."""
+    __tablename__ = "collection_assets"
+
+    id = Column(Integer, primary_key=True, index=True)
+    collection_id = Column(
+        Integer,
+        ForeignKey("asset_collections.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True,
+    )
+    asset_id = Column(
+        Integer,
+        ForeignKey("assets.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True,
+    )
+    sort_order = Column(Integer, default=0, nullable=False)
+    created_at = Column(DateTime(timezone=True), server_default=func.now())
+
+    collection = relationship("AssetCollection", back_populates="items")
+    asset = relationship("Asset")