Add migration 055: display_calendar_id on event_invitations

Adds nullable FK to calendars, index, and backfills accepted/tentative
invitations with each user's default calendar.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/alembic/versions/055_add_display_calendar_to_event_invitations.py

@@ -0,0 +1,51 @@
+"""Add display_calendar_id to event_invitations.
+
+Allows invitees to assign invited events to their own calendars
+for personal organization, color, and visibility control.
+
+Revision ID: 055
+Revises: 054
+"""
+
+from alembic import op
+import sqlalchemy as sa
+
+revision = "055"
+down_revision = "054"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "event_invitations",
+        sa.Column(
+            "display_calendar_id",
+            sa.Integer(),
+            sa.ForeignKey("calendars.id", ondelete="SET NULL"),
+            nullable=True,
+        ),
+    )
+    op.create_index(
+        "ix_event_invitations_display_calendar",
+        "event_invitations",
+        ["display_calendar_id"],
+    )
+
+    # Backfill accepted/tentative invitations with each user's default calendar
+    op.execute("""
+        UPDATE event_invitations
+        SET display_calendar_id = (
+            SELECT c.id FROM calendars c
+            WHERE c.user_id = event_invitations.user_id
+              AND c.is_default = true
+            LIMIT 1
+        )
+        WHERE status IN ('accepted', 'tentative')
+          AND display_calendar_id IS NULL
+    """)
+
+
+def downgrade() -> None:
+    op.drop_index("ix_event_invitations_display_calendar", table_name="event_invitations")
+    op.drop_column("event_invitations", "display_calendar_id")