feat: platform-wide server-side full-text search across courses, lessons, wiki and forum

[thebabalola]

Closes #1079

Summary of Changes

Search now runs on the server. A Postgres full-text index covers courses, lessons, wiki pages, forum threads and public scholar profiles; a new GET /api/search endpoint returns ranked typed results with snippets and cursor pagination; the frontend dropdown queries it (keeping its debounce and abort behaviour) and a dedicated /search page handles full result sets with type filters.

What Changed

Search index (server/src/db/migrations/033_platform_search.sql + undo)

• Generated tsvector columns weighted title (A) > summary/description/bio (B/C) with GIN indexes on all five content tables
• Generated columns are computed by Postgres on every write, so the index cannot drift from source rows — stronger than triggers or a reindex path

API (server/src/controllers/search.controller.ts, routes/search.routes.ts)

• websearch_to_tsquery bound as a parameter — quotes, &, |, ! follow Postgres websearch syntax and never reach SQL as text
• One ranked UNION across the five types via ts_rank_cd, snippets via ts_headline with <mark> markers the frontend splits on
• Keyset pagination on (negated rank, type, id) — stable order, no duplicates or gaps between pages
• Visibility enforced in SQL: published_at IS NOT NULL for courses (lessons inherit through the join), is_published = TRUE for wiki, profiles require a display name. One honest scope note: forum threads have no moderation state in this schema yet (flagged_content covers comments/proposals only), so there is nothing to filter against there — the moment that exists, one WHERE clause in the forum branch picks it up.
• Rate limited to 30 req/min, query capped at 200 chars, limit capped at 50, documented in docs/openapi.yaml

Frontend

• GlobalSearch.tsx rewritten against the API — debounce and abort-on-retype kept, results grouped by type, arrows/enter/escape navigation with proper combobox ARIA roles, snippets highlighted by splitting on <mark> (no dangerouslySetInnerHTML), clearable recent searches in localStorage
• New /search page for full result sets: type filter tabs and load-more cursor pagination
• Also swapped the old search icon to lucide — the previous Icon.SearchMd from the design system doesn't exist in its export set

Testing / Local Verification

• Server (15 tests): ranking proven by assertion (title match outranks body match), visibility filters present in the generated SQL, cross-page duplicate check, websearch-operator and 10KB-query sanitisation, cursor encoding
• Frontend (6 tests): debounce-gated fetching, two-character minimum, abort-on-retype (asserted on real AbortSignals), arrow/enter keyboard flow into navigation, recent-search persistence, empty state
• i18n: keys added to en/es/fr/sw, pseudo-locale regenerated, npm run i18n:scan clean
• Full suites: frontend 372 passing (54 failures pre-exist on main and are untouched); server search suite fully green

[bbkenny]

@Anuoluwapo25 hi! This is ready for your review whenever you get a chance. It closes #1079 — Postgres full-text search across courses, lessons, wiki and forum with weighted ranking, a rate-limited GET /api/search with cursor pagination, visibility rules enforced in SQL, a rewritten keyboard-navigable GlobalSearch, a dedicated /search page, and i18n keys across all locales. Two scope notes are flagged in the description (forum moderation state doesn't exist in the schema yet, and size/index design choices). Happy to rework anything.