feat(contests): add contest_submissions table + SubmitToContest action

[dylanjeffers]

Summary

• Adds the `contest_submissions` table (api repo, migration 0203) keyed on `(contest_id, track_id)`, with secondary indexes on `track_id` and `user_id`. Backs the new "open contest" flow where entries don't have a remix-parent track.
• Defines a new `SubmitToContest` ManageEntity action constant in `indexer/`. Discovery is the writer; see the companion discovery PR.
• Adds `POST /v1/events/:eventId/submit` that signs and relays a `SubmitToContest` / Event ManageEntity tx with metadata `{"track_id": }`.

This change is API-only — it doesn't process the action; that lives in the discovery indexer.

Test plan

• `bash ddl/local-test.sh` runs migration 0203 cleanly (idempotent on re-run).
• `go build ./... && go vet ./...` (already verified locally).
• Hit `POST /v1/events/:eventId/submit` with a valid auth + `{"track_id": "..."}` against a dev stack and confirm the relayed tx lands a row in `contest_submissions` once discovery indexes it.
• Negative paths: missing / non-numeric `track_id`, unknown `eventId`, unauthenticated → 4xx responses.

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