- add period-scoped holiday architecture end-to-end
- model: scope `SchoolHoliday` to `academic_period_id`
- migrations: add holiday-period scoping, academic-period archive lifecycle, and merge migration head
- API: extend holidays with manual CRUD, period validation, duplicate prevention, and overlap merge/conflict handling
- recurrence: regenerate holiday exceptions using period-scoped holiday sets
- improve frontend settings and holiday workflows
- bind holiday import/list/manual CRUD to selected academic period
- show detailed import outcomes (inserted/updated/merged/skipped/conflicts)
- fix file-picker UX (visible selected filename)
- align settings controls/dialogs with defined frontend design rules
- scope appointments/dashboard holiday loading to active period
- add shared date formatting utility
- strengthen academic period lifecycle handling
- add archive/restore/delete flow and backend validations/blocker checks
- extend API client support for lifecycle operations
- release/docs updates and cleanup
- bump user-facing version to `2026.1.0-alpha.15` with new changelog entry
- add tech changelog entry for alpha.15 backend changes
- refactor README to concise index and archive historical implementation docs
- fix Copilot instruction link diagnostics via local `.github` design-rules reference
