Show bounty activity signals

[GHX5T-SOL]

Refs #224.

Summary

• add active attempt, pull-request claim, and last-activity metadata to listed bounties
• render compact activity signals on bounty rows
• cover active/inactive attempts, duplicate claim grouping, cancelled claims, and non-PR claim exclusion

Validation

• mix format --check-formatted lib/algora/bounties/bounties.ex lib/algora_web/components/bounties.ex test/algora/bounties_test.exs
• TEST_DATABASE_URL=postgresql://postgres:postgres@localhost:15432/algora_test mix test test/algora/bounties_test.exs (20 tests, 0 failures; unrelated local Oban sandbox warnings emitted around test lifecycle)
• git diff HEAD~1..HEAD --check
• git diff HEAD~1..HEAD | gitleaks detect --pipe --no-banner --redact (no leaks found)

Notes

• This is a compact list-row signal slice, not the full realtime/trend/sorting request.
• I did not add a component render assertion or run production-like query-plan checks.