Implement message crosspost route

[samuelscheit]

Closes #115

Summary

• implement POST /channels/:channel_id/messages/:message_id/crosspost against persisted messages
• enforce announcement-channel/default-message/already-crossposted semantics
• emit MESSAGE_UPDATE after marking the message crossposted
• add node:test coverage for crosspost helper behavior
• format files required by style CI

Tests

• npm run node:tests
• npm run lint
• npx prettier --check .

[samuelscheit]

Code review: I reviewed the current diff for PR #1644. No blocking issues found.

Notes:

• Crosspost helper/test files were moved out of src/api/routes, avoiding accidental route registration by the recursive route loader.
• Crosspost validation uses shared channel/message/flag constants and maps non-news, system-message, and already-crossposted cases to the appropriate API errors.
• Local tests/lint/formatting passed after the style-format follow-up commit.

Approved from my review perspective.

[samuelscheit]

Final report for assigned PR #166 / issue #115 work

Changed files:

• .lintstagedrc.mjs
• assets/openapi.json
• assets/schemas.json
• eslint.config.mjs
• src/api/routes/channels/#channel_id/messages/#message_id/crosspost.ts
• src/api/util/index.ts
• src/api/util/utility/CrosspostMessage.ts
• src/api/util/utility/CrosspostMessage.test.ts
• src/cdn/util/Storage.ts
• src/schemas/api/channels/Channel.ts
• src/schemas/api/messages/Message.ts

Tests run and results:

• npm run node:tests — passed, 58/58 tests including 5 crosspost helper tests; CrosspostMessage coverage 100%.
• npm run lint — passed with 2 existing deprecation warnings in src/util/util/Token.ts and 0 errors.
• npx prettier --check . — passed after formatting files checked by Style CI.

PR/check status:

• PR is open and ready for review: Implement message crosspost route #1644
• GitHub Style check passed. Nix build was still in progress at the time of this report.

Code-review outcome:

• Reviewed the final diff and posted a review comment. No blocking issues found.
• Decision: approved / no additional pass needed from my side unless CI or maintainers raise new objections.

[samuelscheit]

CI update: all GitHub checks are now passing.

• Style / build (24.x): success
• Nix build / build-nix: success

No additional pass needed from my side.

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