feat: add connections dashboard with background health checks

- Add /connections page showing all integrations with online/offline status
- Add background health checker that polls Evotor API every 10 minutes
- Add is_online and last_checked_at fields to evotor_connections table
- Replace Evotor navbar link with unified Connections link
- Redirect connect/disconnect flows to /connections
- Add Alembic migration for new columns

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

web/migrations/versions/a1b2c3d4e5f6_add_is_online_last_checked_at_to_evotor_connections.py

@@ -0,0 +1,26 @@
+"""add is_online and last_checked_at to evotor_connections
+
+Revision ID: a1b2c3d4e5f6
+Revises: 2c15000e752b
+Create Date: 2026-03-06 00:00:00.000000
+
+"""
+from alembic import op
+import sqlalchemy as sa
+
+revision = 'a1b2c3d4e5f6'
+down_revision = '2c15000e752b'
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.add_column('evotor_connections',
+        sa.Column('is_online', sa.Boolean(), nullable=False, server_default='0'))
+    op.add_column('evotor_connections',
+        sa.Column('last_checked_at', sa.DateTime(), nullable=True))
+
+
+def downgrade() -> None:
+    op.drop_column('evotor_connections', 'last_checked_at')
+    op.drop_column('evotor_connections', 'is_online')