Fix alert delivery timeouts, search filter bugs, and forum anonymous authoring

[sublime247]

Summary

Closes #994,
Closes #995,
Closes #997,
Closes #990.

• Add timeouts and failure visibility to outbound alert delivery HTTP calls #994 — AlertingService and SlackService fired outbound Slack/PagerDuty HTTP calls with no timeout, and SlackService.sendAlert swallowed delivery failures in an empty catch with logging commented out. Adds ALERT_DELIVERY_TIMEOUT_MS to every outbound call, bounded jittered-backoff retry on transient 5xx responses, an alert_delivery_failures_total counter labelled by channel, and replaces the empty catch with Logger.error including the response status.
• Fix broken category filter caused by missing spread operator in SearchService IN clause #995 — SearchService built the category filter with course.category IN (:cats) instead of the spread form IN (:...cats), so TypeORM bound the array as a single scalar parameter and category filtering silently broke (masked by the surrounding catch). Fixes the spread form and implements the level/language filters declared in SearchFilters but never wired up — they weren't backed by any column, so a migration adds them to course (nullable, indexed, mirroring category).
• Stop masking search failures as empty result sets in SearchService #997 — SearchService.search()/getAutoComplete() caught any error — including infrastructure failures — and returned an empty result set with a 200, making outages indistinguishable from a genuine zero-result search. Now rethrows: infrastructure errors become a 503 (ServiceUnavailableException), malformed filter values (Postgres 22xxx data-exception SQLSTATE) become a 400, and both increment search_query_failures_total labelled by failure class. The existing ES→DB fallback path (and its counter) is untouched.
• Stop attributing forum posts and votes to a shared 'anonymous' author identifier #990 — ForumController computed authorId = req.user?.id || 'anonymous' with no guard on any write handler, so every write was attributed to a shared 'anonymous' identity, and ForumService.vote's lookup by (entityType, entityId, authorId) meant every anonymous voter collided on one row — vote tallies were meaningless. Adds JwtAuthGuard to the write handlers (reads stay public), removes the fallback, and adds a migration that purges/flags pre-existing anonymous rows, recomputes affected vote tallies, and adds a foreign key from forum_votes.authorId to users.id.

Test plan

• npx tsc --noEmit clean on all touched files
• npx eslint clean on all touched files
• Unit tests (mocked axios/query-builder) added/updated and passing:
  • src/monitoring/alerting/alerting.service.spec.ts — timeout, retry/backoff, failure counter
  • src/slack.service.spec.ts — timeout, retry, no-more-empty-catch, failure counter
  • src/search/search.service.spec.ts — spread-form SQL/param assertions (single + multi value), 503/400 classification, failure counter
  • src/forum/forum.controller.spec.ts — real HTTP + real JwtAuthGuard (not mocked) proving anonymous writes get 401 and authenticated writes use the real user id, never 'anonymous'
• src/search/search.service.category-filter.integration.spec.ts — exercises the real TypeORM query builder against Postgres (not mocked), seeding courses across two categories; verified it fails against the original unfixed IN (:cats) form and passes against the fix. Excluded from the default npm test run like other *.integration.spec.ts files in this repo — run explicitly against a live database.
• Migration SQL hand-validated against throwaway Postgres tables (purge → tally recompute → flag → type conversion → FK

[RUKAYAT-CODER]

Kindly resolve conflict.
Your PR is not properly linked, only linked to one issue

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