Fix tech stack filtering for exclusive bounties

[GHX5T-SOL]

Closes #173.

Summary:

• apply the selected tech stack filter to exclusive bounties instead of letting them bypass it
• add a regression test covering a visible Rust exclusive bounty being excluded from a Svelte filtered listing

Validation:

• mix format --check-formatted lib/algora/bounties/bounties.ex test/algora/bounties_test.exs
• mix compile --warnings-as-errors
• TEST_DATABASE_URL=ecto://postgres@localhost:55432/algora_test mix test test/algora/bounties_test.exs (20 tests, 0 failures)
• git diff --check
• git diff -- lib/algora/bounties/bounties.ex test/algora/bounties_test.exs | gitleaks stdin --no-banner --redact --timeout 30

Notes:

• Repo-wide mix format --check-formatted reports unrelated existing formatting differences under this local Elixir 1.19.5 / Erlang OTP 28 setup; the changed-file formatter check passes.
• The targeted test run emitted unrelated Oban sandbox and ChromicPDF startup logs, but ExUnit completed successfully with 20 tests and 0 failures.

[CLAassistant]

All committers have signed the CLA.