feat: per-task on/off switches on /sync page for staged rollout

Adds evo_mirror_enabled and vk_mirror_enabled flags to SyncConfig.
Each of the three background tasks (Зеркало Эвотор / Зеркало ВК /
Синхронизация) can now be enabled independently from the /sync page.

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

web/migrations/versions/0009_sync_config_task_flags.py

@@ -0,0 +1,18 @@
+"""Add evo_mirror_enabled and vk_mirror_enabled to sync_configs."""
+revision = "0009"
+down_revision = "0008"
+branch_labels = None
+depends_on = None
+
+import sqlalchemy as sa
+from alembic import op
+
+
+def upgrade():
+    op.add_column("sync_configs", sa.Column("evo_mirror_enabled", sa.Boolean, nullable=False, server_default="0"))
+    op.add_column("sync_configs", sa.Column("vk_mirror_enabled", sa.Boolean, nullable=False, server_default="0"))
+
+
+def downgrade():
+    op.drop_column("sync_configs", "vk_mirror_enabled")
+    op.drop_column("sync_configs", "evo_mirror_enabled")