Add per-room opt-in email notifications on mention (#67)

Lets a member subscribe to email when they're @mentioned in a room
while offline, alongside #66's always-on DM email. Debounced the same
way #66 is (one email per unread burst, not one per mention), and
deliberately scoped to regular rooms only -- DMs already have #66's
automatic offline email with no separate toggle needed.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/alembic/versions/05b28b0a2261_room_email_notifications_opt_in.py

@@ -0,0 +1,32 @@
+"""room email notifications opt-in
+
+Revision ID: 05b28b0a2261
+Revises: e2fc4d65f93e
+Create Date: 2026-08-28 19:34:23.507200
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = '05b28b0a2261'
+down_revision: Union[str, Sequence[str], None] = 'e2fc4d65f93e'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    """Upgrade schema."""
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.add_column('room_memberships', sa.Column('email_notifications', sa.Boolean(), server_default='false', nullable=False))
+    # ### end Alembic commands ###
+
+
+def downgrade() -> None:
+    """Downgrade schema."""
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.drop_column('room_memberships', 'email_notifications')
+    # ### end Alembic commands ###