feat(contribution): add suggestContributionCause mutation

[idoshamun]

What

Adds suggestContributionCause(url, note): EmptyResponse! @auth @contributionEligibility.

Contributors who unlocked the suggest_causes reward can nominate a cause by URL (with an optional note). The suggestion isn't stored — it's posted to the contributions Slack channel for the team to review.

• Resolver enforces the reward's feature access (contribution_suggest_causes); returns FORBIDDEN without it.
• notifyContributionCauseSuggestedSlack posts to the existing webhooks.contributions channel (SLACK_CONTRIBUTIONS_WEBHOOK).
• The Slack send is not swallowed — a failure surfaces as an error to retry, since it's the only place the suggestion lands.

Frontend: dailydotdev/apps#6293.

Notes

• Needs SLACK_CONTRIBUTIONS_WEBHOOK set in the deploy env (no-ops safely if unset).

Test plan

• Posts a suggested cause to Slack for a contributor who unlocked the reward
• Rejects the suggestion (FORBIDDEN) when the reward is not unlocked
• Rejects a malformed URL

[pulumi]

🍹 The Update (preview) for dailydotdev/api/prod (at 10915d2) was successful.

Resource Changes

    Name                                                       Type                           Operation
~   vpc-native-temporal-deployment                             kubernetes:apps/v1:Deployment  update
~   vpc-native-worker-job-deployment                           kubernetes:apps/v1:Deployment  update
~   vpc-native-expire-super-agent-trial-cron                   kubernetes:batch/v1:CronJob    update
~   vpc-native-post-analytics-clickhouse-cron                  kubernetes:batch/v1:CronJob    update
~   vpc-native-update-trending-cron                            kubernetes:batch/v1:CronJob    update
~   vpc-native-personalized-digest-deployment                  kubernetes:apps/v1:Deployment  update
~   vpc-native-user-profile-analytics-clickhouse-cron          kubernetes:batch/v1:CronJob    update
~   vpc-native-materialize-yearly-best-post-archives-cron      kubernetes:batch/v1:CronJob    update
+   vpc-native-api-clickhouse-migration-254821db               kubernetes:batch/v1:Job        create
~   vpc-native-user-profile-analytics-history-clickhouse-cron  kubernetes:batch/v1:CronJob    update
~   vpc-native-clean-channel-highlights-cron                   kubernetes:batch/v1:CronJob    update
~   vpc-native-clean-zombie-user-companies-cron                kubernetes:batch/v1:CronJob    update
~   vpc-native-update-tag-materialized-views-cron              kubernetes:batch/v1:CronJob    update
~   vpc-native-private-deployment                              kubernetes:apps/v1:Deployment  update
~   vpc-native-clean-zombie-users-cron                         kubernetes:batch/v1:CronJob    update
~   vpc-native-clean-zombie-images-cron                        kubernetes:batch/v1:CronJob    update
~   vpc-native-clean-expired-better-auth-sessions-cron         kubernetes:batch/v1:CronJob    update
~   vpc-native-ws-deployment                                   kubernetes:apps/v1:Deployment  update
~   vpc-native-update-achievement-rarity-cron                  kubernetes:batch/v1:CronJob    update
+   vpc-native-api-db-migration-254821db                       kubernetes:batch/v1:Job        create
~   vpc-native-channel-highlights-cron                         kubernetes:batch/v1:CronJob    update
-   vpc-native-api-db-migration-fd115e90                       kubernetes:batch/v1:Job        delete
~   vpc-native-update-source-public-threshold-cron             kubernetes:batch/v1:CronJob    update
~   vpc-native-bg-deployment                                   kubernetes:apps/v1:Deployment  update
~   vpc-native-validate-active-users-cron                      kubernetes:batch/v1:CronJob    update
~   vpc-native-post-lifecycle-state-clickhouse-cron            kubernetes:batch/v1:CronJob    update
~   vpc-native-update-current-streak-cron                      kubernetes:batch/v1:CronJob    update
~   vpc-native-rotate-daily-quests-cron                        kubernetes:batch/v1:CronJob    update
~   vpc-native-calculate-top-readers-cron                      kubernetes:batch/v1:CronJob    update
~   vpc-native-generate-search-invites-cron                    kubernetes:batch/v1:CronJob    update
~   vpc-native-rotate-weekly-quests-cron                       kubernetes:batch/v1:CronJob    update
~   vpc-native-sync-subscription-with-cio-cron                 kubernetes:batch/v1:CronJob    update
~   vpc-native-subscription-anniversary-achievements-cron      kubernetes:batch/v1:CronJob    update
~   vpc-native-daily-digest-cron                               kubernetes:batch/v1:CronJob    update
~   vpc-native-clean-old-notifications-cron                    kubernetes:batch/v1:CronJob    update
~   vpc-native-update-highlighted-views-cron                   kubernetes:batch/v1:CronJob    update
~   vpc-native-squad-posts-analytics-refresh-cron              kubernetes:batch/v1:CronJob    update
~   vpc-native-clean-gifted-plus-cron                          kubernetes:batch/v1:CronJob    update
~   vpc-native-user-profile-updated-sync-cron                  kubernetes:batch/v1:CronJob    update
~   vpc-native-update-tags-str-cron                            kubernetes:batch/v1:CronJob    update
~   vpc-native-personalized-digest-cron                        kubernetes:batch/v1:CronJob    update
... and 15 other changes