fix(gateway): boot-seed backends so site-api survives restart

[echobt]

Summary

• Prod chain.joinbase.ai /v1/site/* went empty after a 12:38 UTC gateway+prism image pull: the in-memory challenge registry was wiped, so site-api soft-failed while Postgres still had 488 Prism submissions and 11k Design runs.
• Gateway now boot-seeds prism + design from BASE_GATEWAY_BACKENDS (compose default), so a restart cannot leave /challenge/* at 503.
• Migration 0020 allows scoring_mode=benchmarks on prism_eval_run (v4 finalize was hitting prism_eval_run_mode_check).
• Adds base-reseed-backends.timer as a 2-minute fallback for older images (already enabled on the prod master).

Test plan

• Prod reseed: /v1/site/arenas/prism/leaderboard returns epoch 24519 / 30 rows; design 340; network 482 agents
• cargo test -p gateway-registry --lib (parse/seed + existing registry tests)
• cargo test -p gateway --lib
• CI green on this PR
• After merge/tag: confirm a gateway restart still serves /challenge/prism/health 200 without a manual POST

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📥 Commits

Reviewing files that changed from the base of the PR and between c3377e9 and fcbd012.

📒 Files selected for processing (11)

• crates/db/migrations/0020_prism_eval_scoring_benchmarks.sql
• crates/gateway-registry/src/lib.rs
• crates/gateway/src/lib.rs
• deploy/AGENTS.md
• deploy/scripts/prod-reseed-backends.sh
• deploy/scripts/register-challenge-backends.sh
• deploy/scripts/remote-deploy.sh
• deploy/systemd/base-reseed-backends.service
• deploy/systemd/base-reseed-backends.timer
• docker-compose.yml
• docs/SITE_API.md

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