V51: Cart items unique constraint fix + User deletion FK constraints

Part 1: Cart Items Unique Constraint

The original constraint only checked (cart_id, product_id), preventing users from adding the same product with different options to their cart.

New constraint uses MD5 hash of selected_specs JSONB for efficient unique checking across all option combinations.

Part 2: User Deletion Foreign Key Constraints

Changes ON DELETE behavior for user-related tables to support GDPR compliance - preserve financial/support records while allowing user deletion.

Changes

• Drops existing idx_shop_cart_items_unique index
• Creates new unique index including MD5 hash of selected_specs
• orders.user_id: RESTRICT → SET NULL (preserve orders, anonymize user)
• billing_profiles.user_id: CASCADE → SET NULL (preserve for history)
• tickets.user_id: DELETE_ALL → SET NULL (preserve support history)
• orders.user_id: DROP NOT NULL constraint
• billing_profiles.user_id: DROP NOT NULL constraint
• tickets.user_id: DROP NOT NULL constraint