fix: improve quality issue tracing and evaluation fallback

[fj-sunny]

Closes #145

Changes

• Count affected users by user_id or anonymous_id.
• Include authenticated user IDs in backend quality events.
• Gracefully preserve transcript when audio is unavailable.
• Avoid realtime-channel close race errors.

Verification

• Backend compile passed.
• Quality controller and telemetry tests passed.
• Web realtime event checks and production build passed.

[fennoai]

The changed contracts for quality-user attribution, anonymous-user aggregation, transcript-preserving evaluation fallback, and realtime-channel close races are consistent with their callers and persistence/reporting consumers. I found no actionable correctness, reliability, or compatibility issue in the fixed PR diff.

Verification: backend ./mvnw -q -DskipTests compile passed; npm run check:realtime-events passed. The web production build could not run because the local vite executable is not executable (vite: Permission denied).

[HansonL622]

LGTM