fix: distinguish missing baseline from Site API fetch error

[veksen]

Summary

• fetchPreviousRun now returns a PreviousRunResult discriminated union (found / not-found / error) so callers can tell a genuine 404 ("no baseline on branch") apart from a 5xx, timeout, or network failure.
• main.ts emits the existing "No baseline found... add a push trigger" guidance only on not-found, and a new warning with the actual reason on error ("Failed to fetch baseline... likely a transient Site API issue — re-run the check to retry.").
• Previously both cases produced the same misleading "No baseline found" log, so a 502 from the Site API looked identical to an actually-missing baseline.

Test plan

• npm run typecheck — clean
• npx vitest run — 161/161 passing

🤖 Generated with Claude Code

[github-actions]

Query Doctor Analysis

View full run details

2 queries analyzed

2 pre-existing issues

• SELECT "guests"."id", "guests"."session_id", "guests"."username", "guests"."avatar_path", "guests"."color", "guests"."side", "guests"."audio_recording_path", "guests"."audio_recording_public", "gue...
  index assets(event_id, inserted_at desc)
  cost 31,003,449 → 1,493 (100% reduction)
• SELECT * FROM guest_ip_addresses WHERE ip_address = '127.0.0.1';
  index guest_ip_addresses(ip_address)
  cost 154,402 → 8 (100% reduction)

_{Using assumed statistics (10000000 rows/table). For better results, sync production stats.}