Story: Model market data cuts as proper ORE Studio reference data

A "cut" identifies which time-of-day market data snapshot is used for official valuation (e.g. "NY 10am", "London 4pm"). It's referenced conceptually across the codebase (see Pricing Configuration) but has no domain model anywhere — no entity, no table, no FK target. Currency pair support in reference data needs a default_cut_code on its currency_pair entity and, rather than leave it as unvalidated free text, splits proper cut modelling off into this story: a small lookup entity (code, name, description, cut time, timezone) that currency_pair and anything else needing a cut can soft-FK against.

• [ ] A market_data_cut (or similarly named) lookup entity exists — same shape as business_day_convention_type=/=currency_pair_classification (code, name, description, display_order), plus cut-specific attributes (cut time, timezone).
• [ ] currency_pair gets a default_cut_code field (soft or hard FK to the new entity) added by this story — it does not exist on currency_pair today. It was dropped entirely from that entity's design rather than kept as interim free text, since nothing currently consumes it; add it back here once there's something to FK against.
• [ ] Any other cut-code field discovered along the way is migrated onto the same FK.
• [ ] Qt CRUD screens exist for the new entity, following the standard aux-type pattern.