feat(backend): implement BE-136, BE-131, BE-130, BE-129

[Mkalbani]

Closes #966
Closes #990
Closes #989
Closes #988

Summary

This PR implements the four backend hardening tickets together because they all touch request handling, logging, and configuration.

BE-136 — Add request correlation IDs to Winston logging

• Added CorrelationIdMiddleware to generate or honor inbound X-Request-Id, attach it to the request, and echo it back on responses.
• Added AsyncLocalStorage based correlationId.storage.ts so any log line can pick up the current request ID without threading it through every call.
• Updated Winston formatting to inject requestId into every JSON log line.
• Propagated the correlation ID into BullMQ job payloads and restored it inside DocumentProcessor workers.
• Included requestId in HttpExceptionFilter error responses.
• Added redaction for authorization, cookie, password, token, and any *_token / *_secret / *_password / *_key fields.

BE-131 — Document secrets management and rotation procedure

• Added backend/docs/SECRETS.md.
• Inventoried every backend secret with purpose and blast radius.
• Documented storage per environment (local .env, platform secret manager, CI secrets).
• Wrote rotation runbooks for JWT, database, Redis, OAuth, mail, and Stellar secrets.
• Included a JWT key-id / multi-secret grace-period approach so existing sessions survive rotation.
• Defined rotation cadence, incident response steps, and contributor dev-credential guidance.

BE-130 — Validate uploaded document files by content

• Added FileValidationPipe with max-size enforcement, MIME allowlist, and magic-byte sniffing.
• Rejects archives, executables, and any declared MIME that does not match actual content.
• Detects PDF active content (embedded files, page actions) via pdf-lib.
• Strips image metadata with sharp.
• Switched document storage to randomized UUID keys so the client filename is never a path component.
• Added GET /documents/:id/download serving with Content-Disposition: attachment, correct Content-Type, and X-Content-Type-Options: nosniff.

BE-129 — Harden CORS configuration

• Replaced the loose FRONTEND_URL || localhost origin with an exact-origin allowlist.
• Production startup now throws if FRONTEND_URL is missing.
• Supports comma-separated FRONTEND_URL values.
• Restricted allowed methods and headers to what the frontend actually uses.
• Updated OAuth redirect helper to use the same allowlist-aware resolver.

Verification

• cd backend && npx tsc --noEmit passes.
• cd backend && npx jest --no-coverage passes: 45 tests, 12 suites.

Notes for reviewers

• sharp was added for image metadata stripping; a small local type declaration (backend/src/types/sharp.d.ts) is included because the installed sharp types resolve incorrectly under the project's moduleResolution: node setting.
• file-type was not added; magic-byte detection is implemented inline to keep dependencies minimal and avoid ESM/CJS interop issues.

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[mftee]

Very good job