Fix join-code redemption without collection-group dependency

[nwang783]

Summary

• add a top-level joinCodeLookups/{codeHash} document when issuing join codes so redemption no longer depends on a Firestore collection-group query
• keep a legacy fallback for older join-code docs and backfill the lookup document on successful redemption
• declare the missing Firestore collection-group index for joinCodes.codeHash so existing production join codes continue to work after deploy

Verification

• node --test test/join-flow-api.test.cjs
• npm test
• npm run test:dashboard
• npm run build:dashboard
• npm run dev:up
• npm run dev:seed
• curl -s -X POST http://127.0.0.1:5001/nomergeconflicts/us-central1/api/teams/join -H 'content-type: application/json' --data '{"joinCode":"DLAP-YVYH-CGH9","seatName":"teammate-local-check"}'
• verified in Firestore emulator that joinCodeLookups/<sha256(joinCode)> exists and teams/<teamId>/memberships contains teammate-local-check

Adversarial Review

• checked backward compatibility for legacy join codes: old docs still redeem through the fallback path and now self-heal by writing the lookup doc
• checked the primary risk path for production: existing join codes still need the new Firestore index deployed, which is included in this diff
• checked failure behavior for stale lookup docs: the transaction now rejects missing join-code docs as invalid instead of creating a seat from partial state