Feature: Scheduled menu-item offers by time, date, and weekday

[itsbkm]

Problem or motivation\n\nCafes often run item-level promotions such as happy-hour pricing, weekday specials, and month-specific sales. Staff should not have to remember to apply these discounts manually, and offers should activate and expire automatically.\n\n## Current behavior\n\nFloCafe has manual checkout discounts and a broader promotion-engine proposal in #359, but there is not yet a focused workflow for managing scheduled offers directly against menu items.\n\n## Desired outcome\n\nAdd menu-item offers that automatically apply a discount when the qualifying item is added to an order or the order is priced.\n\nAn offer should support:\n\n- A percentage or fixed-amount discount.\n- One or more specific menu items, with clear behavior for variants/add-ons if applicable.\n- A start and end date/time, including AM/PM-aware local store time.\n- Recurring weekday restrictions (for example, Monday-Friday).\n- Month-specific or date-range availability.\n- Active/inactive status and a clear preview of the effective schedule.\n- Conflict handling when multiple offers match the same item (priority, stacking rules, or another explicit deterministic rule).\n\nExamples:\n\n- 20% off a particular item from 3:00 PM to 5:00 PM.\n- A breakfast item offer valid only on weekdays.\n- A seasonal offer active during a selected month/date range.\n\nThe final bill should clearly show the applied offer and discount.\n\n## Proposed technical approach\n\nTreat this as the menu-item/scheduling detail of the promotion engine proposed in #359, reusing its promotion, adjustment, priority, stacking, and historical-snapshot concepts rather than creating a separate discount system.\n\n- Use the configured store timezone for schedule evaluation.\n- Evaluate offers through backend-authoritative pricing.\n- Store an applied-offer snapshot on the order so later edits do not change historical bills.\n- Ensure offline POS operation continues to work with locally stored schedules.\n- Define whether offers can combine with customer-type discounts, manual discounts, coupons, or other promotions.\n\n## Acceptance criteria\n\n- [ ] Authorized users can create, edit, activate, deactivate, and delete/archive menu-item offers.\n- [ ] Offers can target individual menu items and define percentage or fixed-amount discounts.\n- [ ] Offers support start/end date and time in the configured store timezone.\n- [ ] Offers support weekday restrictions and date/month ranges.\n- [ ] Matching offers are applied automatically without staff re-entering a discount.\n- [ ] Multiple matching offers resolve deterministically according to documented priority/stacking rules.\n- [ ] The checkout and receipt show the applied offer and resulting discount.\n- [ ] Historical orders retain the applied-offer snapshot.\n- [ ] Offer evaluation works offline using the locally available catalog and schedule.\n- [ ] Interaction with customer-type, manual, and coupon discounts is documented and tested.\n\n## Additional context\n\nThis is a planned enhancement for later prioritization. See #359 for the broader promotion-engine boundary and avoid implementing a parallel promotion model.

[carvalab]

Heads-up: posted a boundary + stacking approval question on #359 (comment above). #632 implementation should wait for maintainer confirmation there before committing to a discount model.