Use POST for Discord JWT request

[raymondjacobson]

Summary

• send the Discord JWT request as POST /request_discord_code with a JSON body instead of query params
• encode the Discord OAuth state payload before appending it to the OAuth URL

Context

PR #14520 fixed the mixed-content issue by switching the production Discord bot URL to HTTPS. This follow-up moves signature/message transport out of the URL and into a JSON request body, avoiding URL length and query logging concerns.

Companion PRs

• Bot POST support: https://github.com/AudiusProject/discord-audio-bot/pull/24
• k8s JWT secret wiring: https://github.com/AudiusProject/audius-k8s/pull/514

Rollout

Merge/deploy after the bot supports POST. Recommended order:

1. Create/set discord-bot-JWT_SECRET in GCP Secret Manager.
2. Merge/apply the k8s secret wiring PR.
3. Merge/deploy the bot POST-support PR.
4. Merge/deploy this apps PR.

Test plan

• git diff --check

Could not run local Vitest because this worktree has no installed node_modules, so vitest is unavailable (sh: vitest: command not found).

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Latest commit: e636860

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