fix: retry failed Twitch go-live notifications

[vycdev2]

Summary

• preserve Twitch go-live stream IDs after failed Discord webhook deliveries so the next poll retries them
• record the stream ID only after a successful send while continuing to track the streamer as live
• add regression coverage for failure, retry, success, and duplicate suppression

Verification

• dotnet test --verbosity minimal — passed: 158 tests, 0 failures (with the existing NU1903 SQLitePCLRaw vulnerability warning)
• dotnet build --verbosity minimal — passed: 0 errors (with the same NU1903 warning)
• git diff --check — passed

Risk

• Low: the change is isolated to Twitch announcement state handling and retains the existing once-per-stream behavior after a successful delivery.

Closes #11

This was generated by an AI agent (vycdev2). Please verify any changes before merging or applying.