Define challenge forum discussion on create (opportunities app forums)

[jmgasper]

Summary

Challenge forums are moving to the new forums implementation in the opportunities app, and the challenge-forum-processor flow (create a Vanilla forum, then write the URL back into discussions[].url) is being fully deprecated.

With this change the API defines the discussions object itself when a challenge is created:

• provider: topcoder (was vanilla)
• type: CHALLENGE
• name: the challenge name, truncated to FORUM_TITLE_LENGTH_LIMIT
• url: <OPPORTUNITIES_CHALLENGE_URL>/<challengeId>?tab=forum, e.g. https://www.topcoder.com/opportunities/challenge/6a5da7b6-3841-43cb-ae9d-98416bea0d9d?tab=forum

The challenge id is generated up front so the URL is known before the record is written. The challenge.notification.create bus event is still raised and now carries the completed discussion, so nothing downstream needs to fill it in. Any discussions supplied by the caller on create are replaced by the API-defined one (the create request schema still accepts them for backward compatibility).

Config

New OPPORTUNITIES_CHALLENGE_URL (documented in the README). It follows the repo convention of defaulting to the dev host (https://www.topcoder-dev.com/opportunities/challenge), so production needs:

OPPORTUNITIES_CHALLENGE_URL=https://www.topcoder.com/opportunities/challenge

Not changed

• The update flow's "only M2M can change discussion url/options" rule is untouched.
• src/scripts/seed/Challenge.json still contains Vanilla discussion fixtures.

Testing

• pnpm lint and tsc -p tsconfig.build.json --noEmit pass.
• Added a unit test that creates a challenge and asserts the returned discussion, the persisted ChallengeDiscussion row, and the challenge.notification.create bus payload all carry the topcoder provider and the ?tab=forum URL. Run against a local Postgres: it passes, and the remaining create-challenge failures in that file are identical on develop (fixture uses lowercase type: "challenge", plus unconfigured external service URLs).

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