fix(ci): unblock invite-gate and add backend platform refs

[AquiGorka]

Summary

Two changes bundled to unblock invite-gate CI and round out the reusable workflow inputs:

1. Pre-create the BadgerDB trace dir. The invite-gate workflow has been red since 2026-04-22. The jaeger container (non-root) couldn't write to the ./.traces/invite-gate bind mount because CI invokes docker compose up directly and skips the mkdir + chmod 777 that test.sh does locally. The workflow now replicates that prep before compose up.
2. Add provider_platform_ref, council_platform_ref, pay_platform_ref inputs. Mirrors the existing *_console_ref inputs so each backend repo's PR can run invite-gate against its own branch (vs. always main).

Test plan

• Ran ./test.sh invite-gate locally — all 6 invite-gate tests pass (provider-console, council-console, moonlight-pay × blocked + allowlisted, ~3.7m).
• Verified jaeger container is Healthy after mkdir -p .traces/invite-gate && chmod 777 step (previously exited 1 on permission denied).
• deno fmt + deno lint clean on changed file.
• CI run on this PR will be the live verification — expect green.

Before/after

The workflow's run: step now begins:

set +e
mkdir -p .traces/invite-gate
chmod 777 .traces/invite-gate
docker compose -f docker-compose.invite-gate.yml up -d

And the three new inputs default to main so existing callers (provider-console, council-console, moonlight-pay) keep working unchanged.

Follow-ups

The three backend repos (provider-platform, council-platform, pay-platform) each get a caller workflow in their own PRs that reference this reusable workflow at @main after merge.