chestshop_shop: per-shop search visibility flag (owner can hide from /find)

Part of PAR-166 (spin out /find). Data-layer gap.

Legacy chestshop-database has a per-shop visible BOOLEAN so an owner can hide a shop from /find search results (independent of it being open/destroyed). Treasury's chestshop_shop only has active (destroyed-or-not) — there is no owner-controlled visibility.

Do:

• Add a visible TINYINT(1) NOT NULL DEFAULT 1 column to chestshop_shop via a new economy-schema V<n>__*.sql migration (do NOT edit V6).
• Add a MarketApi.updateShopVisibility(world, x, y, z, boolean) method + mapper write path in ChestShopMarketMapper/MarketApiImpl.
• Plumb visible through ChestShopShopRecord so upsertShop preserves it (don't reset to default on restock/price-change upserts).
• Read side: expose visible (and let /api/v1/chestshop/shops filter on it) so the spun-out /find respects it.
• The owner-facing toggle command itself lives in the new plugin (sub-issue of PAR-166); this issue is just the data layer + write API.

Note: distinct from active. active=0 = sign destroyed (kept for history); visible=0 = live shop the owner chose to hide.