feat: /sync settings page with price multiplier and description postfix

Adds price_multiplier and description_postfix to SyncConfig. The sync
page at /sync lets users configure them. vk_sync reads these per-user
settings and applies the multiplier to price and appends the postfix
as "(postfix)" to the VK product description.

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

web/migrations/versions/0008_sync_config_price_postfix.py

@@ -0,0 +1,18 @@
+"""Add price_multiplier and description_postfix to sync_configs."""
+revision = "0008"
+down_revision = "0007"
+branch_labels = None
+depends_on = None
+
+import sqlalchemy as sa
+from alembic import op
+
+
+def upgrade():
+    op.add_column("sync_configs", sa.Column("price_multiplier", sa.Numeric(10, 4), nullable=False, server_default="1.0"))
+    op.add_column("sync_configs", sa.Column("description_postfix", sa.String(255), nullable=True))
+
+
+def downgrade():
+    op.drop_column("sync_configs", "description_postfix")
+    op.drop_column("sync_configs", "price_multiplier")