feat: 2026.1.0-alpha.16 – dashboard banner refactor, period auto-activation, text & docs

Dashboard (dashboard/src/dashboard.tsx, settings.tsx, apiAcademicPeriods.ts):
- Refactor loadHolidayStatus to useCallback with stable empty-deps reference;
  removes location.pathname dependency that caused overlapping API calls at mount
  and left the banner unresolved via request-sequence cancellation
- Add key prop derived from severity:text to Syncfusion MessageComponent to force
  remount on state change, fixing stale banner that ignored React prop/children updates
- Correct German transliterated text to proper Umlauts throughout visible UI strings
  (fuer -> für, oe -> ö, ae -> ä etc. across dashboard and settings views)

Backend (server/init_academic_periods.py):
- Refactor to idempotent two-phase flow: seed default periods only when table is
  empty; on every run activate exactly the non-archived period covering date.today()
- Enforces single-active invariant by deactivating all periods before promoting match
- Emits explicit warning when no period covers current date instead of doing nothing

Deployment (docker-compose.prod.yml):
- Add init_academic_periods.py to server startup chain after migrations and defaults;
  eliminates manual post-deploy step to set an active academic period

Release docs:
- program-info.json: bump to 2026.1.0-alpha.16; fix JSON parse error caused by
  typographic curly quotes in the new changelog entry
- TECH-CHANGELOG.md: detailed alpha.16 section with root-cause motivation for both
  dashboard refactoring decisions (unstable callback ref + Syncfusion stale render)
- DEV-CHANGELOG.md: document dashboard refactor, Syncfusion key fix, Umlaut changes,
  and program-info JSON regression and fix
- README.md: add Latest Release Highlights section for alpha.16
- .github/copilot-instructions.md: sync file map, prod bootstrap note, backend and
  frontend pattern additions for academic period init and Syncfusion remount pattern

DEV-CHANGELOG.md

@@ -5,6 +5,10 @@ This changelog tracks all changes made in the development workspace, including i
 ---
 
 ## Unreleased (development workspace)
+- Programminfo GUI regression/fix: `dashboard/public/program-info.json` could not be loaded in Programminfo menu due to invalid JSON in the new alpha.16 changelog line (malformed quote in a text entry). Fixed JSON entry and verified file parses correctly again.
+- Dashboard holiday banner fix: `dashboard/src/dashboard.tsx` — `loadHolidayStatus` now uses a stable `useCallback` with empty deps, preventing repeated re-creation on render. `useEffect` depends only on the stable callback reference.
+- Dashboard Syncfusion stale-render fix: `MessageComponent` in the holiday banner now receives `key={`${severity}:${text}`}` to force remount when severity or text changes; without this Syncfusion cached stale DOM and the banner did not update reactively.
+- Dashboard German text: Replaced transliterated forms (ae/oe/ue) with correct Umlauts throughout visible dashboard UI strings — `Präsentation`, `für`, `prüfen`, `Ferienüberschneidungen`, `verfügbar`, `Vorfälle`, `Ausfälle`.
 - TV power intent (Phase 1): Scheduler publishes retained QoS1 group-level intents to `infoscreen/groups/{group_id}/power/intent` with transition+heartbeat semantics, startup/reconnect republish, and poll-based expiry (`max(3 × poll_interval_sec, 90s)`).
 - TV power validation: Added unit/integration/canary coverage in `scheduler/test_power_intent_utils.py`, `scheduler/test_power_intent_scheduler.py`, and `test_power_intent_canary.py`.
 - Monitoring system completion: End-to-end monitoring pipeline is active (MQTT logs/health → listener persistence → monitoring APIs → superadmin dashboard).