feat: add async federation export trigger

[mashbean]

Summary

• add lambda_async federation export trigger mode for publish/edit paths
• keep off and record_only behavior unchanged; off still means no trigger call
• enforce strict public-only + author/article federation gate before Lambda invocation
• invoke federation-export Lambda asynchronously only for eligible rows, with S3 output required
• record queued, skipped, or failed status in federation_export_event

Safety

• this PR does not enable production outbound by itself
• rollback remains setting MATTERS_FEDERATION_EXPORT_TRIGGER_MODE=record_only or off
• missing Lambda function name or S3 bucket marks the event failed and is caught by existing publish/edit error handling
• private/paywalled/non-opt-in articles are recorded as skipped and do not invoke Lambda

Required env for lambda_async

• MATTERS_FEDERATION_EXPORT_TRIGGER_MODE=lambda_async
• MATTERS_FEDERATION_EXPORT_LAMBDA_FUNCTION_NAME
• MATTERS_FEDERATION_EXPORT_S3_BUCKET
• optional: MATTERS_FEDERATION_EXPORT_S3_PREFIX
• optional: MATTERS_FEDERATION_WEBF_DOMAIN

Verification

• npm run build
• npm run lint
• npx -p node@18 node --experimental-vm-modules --no-experimental-fetch node_modules/.bin/jest build/connectors/__test__/federationExportService.test.js build/mutations/article/__test__/federationExportTrigger.test.js --runInBand --forceExit

Notes

• local Node is v24, so Jest was run through temporary Node 18 to match repo engine.
• npm install reported existing audit findings; no audit fix was run to avoid unrelated dependency churn.

[codecov]

Codecov Report

❌ Patch coverage is 76.31579% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.41%. Comparing base (85bdbe1) to head (21b6358).
⚠️ Report is 19 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/connectors/article/federationExportService.ts	72.72%	9 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #4840      +/-   ##
===========================================
- Coverage    72.82%   72.41%   -0.42%     
===========================================
  Files         1054     1054              
  Lines        21263    20946     -317     
  Branches      4671     4538     -133     
===========================================
- Hits         15485    15168     -317     
- Misses        5706     5708       +2     
+ Partials        72       70       -2     

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