Fix notifications fleet alert migration for MySQL and add UI fields

- Change alertscope from JSONB to TEXT for MySQL compatibility
- Add Alert Fleet checkbox, scope, and timeout fields to notification form
- Fields appear conditionally when Alert Fleet is checked

plugins/notifications/migrations/versions/0006_add_fleet_alerts.py

@@ -5,7 +5,7 @@ to support fleet-wide alert broadcasting. Creates notificationacknowledgements
 table to track which assets have acknowledged fleet alerts.
 
 Fleet alerts can be scoped to specific asset types, locations, or business units
-via the alertscope JSONB field. alerttimeout=0 means no auto-dismiss (manual only).
+via the alertscope TEXT field (JSON string). alerttimeout=0 means no auto-dismiss (manual only).
 
 Idempotent: column and table creation are skipped when they already exist.
 
@@ -14,7 +14,6 @@ Revises: notifications0005boardorder
 """
 from alembic import op
 import sqlalchemy as sa
-from sqlalchemy.dialects.postgresql import JSONB
 
 
 revision = 'notifications0006fleetalerts'
@@ -43,7 +42,7 @@ def upgrade():
 
         if 'alertscope' not in cols:
             op.add_column('notifications',
-                          sa.Column('alertscope', JSONB, nullable=True))
+                          sa.Column('alertscope', sa.Text, nullable=True))
 
         if 'alerttimeout' not in cols:
             op.add_column('notifications',