[Chore] Harden suggested task reaction launches

[roomote-roomote]

Opened on behalf of Matt Rubens. Follow up by mentioning @roomote-roomote, in the web UI, or in Slack.

What changed

• Require Slack users to have an active linked Roomote account before a suggestion reaction can claim or launch work.
• Add concurrent duplicate-reaction coverage for Slack, Telegram, and Teams that verifies only one task launch reaches the task creator.
• Exercise the shared work-item claim CAS and surface-specific Telegram and Teams resolvers with real concurrent database claims.

Why this change was made

Suggestion reactions should behave consistently across chat surfaces and remain idempotent when duplicate webhook deliveries or simultaneous reactions contend for the same work item.

Impact

Linked users continue to launch suggestions normally, while unlinked Slack users now receive a link-account prompt without consuming the suggestion. Concurrent reactions produce one winning launch and leave duplicate attempts as no-ops. Focused tests and the repository pre-push checks pass; the full API ESLint command remains blocked by five pre-existing unused-disable warnings in unrelated tests.

[roomote-community]

No code issues found. See task

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