iEduPortal API v1 — Stage 19

Adds protected read-only announcements retrieval at /api/v1/announcements/.
No database schema changes. Existing API authentication/configuration is preserved.

Pupil/student: own-class and global announcements.
Parent: announcements for classes containing their children, plus global announcements.
Admin/staff roles: permitted to retrieve announcements; class_id/date filters available.
