Add SetShare model for per-user set sharing

app/models/set_share.py

@@ -0,0 +1,31 @@
+"""
+Explicit share: one user granted access to one set whose visibility
+is 'shared'.
+
+Created by: Jess Rogerson (yelling commands at Claude.AI)
+"""
+
+from datetime import datetime
+
+from app import db
+
+
+class SetShare(db.Model):
+    """A set shared with a specific user."""
+
+    __tablename__ = 'set_shares'
+    __table_args__ = (
+        db.UniqueConstraint('set_id', 'user_id', name='uq_set_share'),
+    )
+
+    id = db.Column(db.Integer, primary_key=True)
+    set_id = db.Column(db.Integer, db.ForeignKey('sets.id', ondelete='CASCADE'),
+                       nullable=False, index=True)
+    user_id = db.Column(db.Integer, db.ForeignKey('users.id', ondelete='CASCADE'),
+                        nullable=False, index=True)
+    created_at = db.Column(db.DateTime, nullable=False, default=datetime.utcnow)
+
+    user = db.relationship('User', lazy='joined')
+
+    def __repr__(self):
+        return f'<SetShare set={self.set_id} user={self.user_id}>'