feat: implement lazy-link for pending invites and add hub_invite_token to InviteToken model

Co-authored-by: Copilot <copilot@github.com>

backend/alembic/versions/84daf7ae73cd_add_hub_invite_token_to_invite_tokens.py

@@ -0,0 +1,26 @@
+"""add_hub_invite_token_to_invite_tokens
+
+Revision ID: 84daf7ae73cd
+Revises: b2c3d4e5f6a7
+Create Date: 2026-05-07 14:33:14.793300
+"""
+from typing import Sequence, Union
+from alembic import op
+import sqlalchemy as sa
+
+revision: str = '84daf7ae73cd'
+down_revision: Union[str, Sequence[str], None] = 'b2c3d4e5f6a7'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        'invite_tokens',
+        sa.Column('hub_invite_token', sa.String(length=255), nullable=True),
+        schema='core',
+    )
+
+
+def downgrade() -> None:
+    op.drop_column('invite_tokens', 'hub_invite_token', schema='core')