feat: course prerequisites, announcements, admin dashboard, course import

[codemagician1949]

Heads up: based on the build-fix branch

This branch is based on #444 (main currently fails to build — 3 syntax errors from a recent merge, unrelated to this batch). Until #444 merges, this PR's diff will also show those 3 files (course.routes.ts, user.routes.ts, admin-users.service.ts) — that's the build fix, not this feature work. Once #444 merges and this branch is rebased/main is pulled in, that noise will disappear from the diff.

#354 — GET /api/v1/courses/:id/prerequisites

• courses.prerequisites: new jsonb string[] column (migration 0020), admin-configurable via the existing POST/PUT /admin/courses (a course can't be its own prerequisite — filtered on update).
• getCoursePrerequisites() resolves the configured prerequisite courses and annotates each with the caller's completion status (enrollments.completedAt IS NOT NULL), plus an overall met flag. Anonymous callers see every prerequisite as incomplete.

#353 — Announcements

• New announcements table (migration 0021): title, message, priority (normal/high/urgent), active, createdAt, expiresAt.
• New module (src/modules/announcements/): full admin CRUD (POST/GET/PUT/DELETE /admin/announcements, admin-guarded) plus a public GET /announcements filtered to active && (no expiry || expiry in the future).

#367 — GET /api/v1/admin/dashboard

• New module (src/modules/admin/dashboard.*): total users, new users (today/week/month), total enrollments, quiz completion rate (graded ÷ total submissions), total credentials, total rewards claimed (SUM(rewardAmount) where claimed), plus week-over-week trend comparisons for new users and enrollments. Cached 5 minutes via the existing cache/index.ts helpers. Admin only.

#366 — Course import

• POST /api/v1/admin/courses/import — multipart/form-data with one JSON file part (reuses the already-registered @fastify/multipart), validated against a Zod schema (core course fields + an optional modules array), creates the course via the existing createCourse() path, then creates each module via the existing createModule() so IDs/locking/audit/cache-invalidation all match a manually-created module. Returns { courseId, modulesCreated }.

  Deviation from the issue, flagging explicitly: the issue's literal route is /admin/courses/:id/import, but its own scope and acceptance criteria describe creating a new course from the file ("creates the course... returns the created course ID") — a target :id doesn't fit that (there's no existing course to import into). Implemented as the collection-level /admin/courses/import instead, matching the actual described behavior. Happy to add a path param back if the real intent was importing additional content into an existing course — wasn't sure which was meant, said so rather than guessing silently.

Verification

• npm run build (tsc) is clean.
• No new automated tests for these four endpoints — flagging honestly rather than claiming coverage that isn't there. Ran out of runway to write and verify the DB-mock-based unit tests this repo's existing suite uses (tests/unit/courses/*.test.ts pattern). The new logic (prerequisite resolution, announcement active-filtering, dashboard aggregates, import parsing) has no dedicated test coverage yet — worth adding before/as part of review.

Closes #354
Closes #353
Closes #367
Closes #366

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