[ENHANCEMENT] Add Sentry or equivalent error tracking integration

[kilodesodiq-arch]

Engineering Gap

No centralized error tracking or alerting. Production errors discovered only through user reports. Error handling exists (exception filter, error boundaries) but no aggregation or alerting.

Codebase Evidence

• app/backend/src/common/filters/http-exception.filter.ts: Catches all exceptions but only logs locally
• app/frontend/src/components/ErrorBoundary.tsx: Catches React errors but only console.error
• No Sentry, DataDog, or similar SDK in any package.json

Risk Profile

Production errors invisible. User-reported bugs only. No error trend analysis.

Remediation Strategy

Add Sentry SDK to backend, frontend, and mobile. Configure error sampling, PII scrubbing, and environment separation. Add alerting for error rate spikes.

Success Conditions

• Sentry SDK added to backend
• Sentry SDK added to frontend
• Error sampling configured
• PII scrubbing configured

Change Surface

Files: package.json files, main.ts, ErrorBoundary.tsx, env configuration

Security Review

Error data must be scrubbed of PII before sending to Sentry.

Completion Checklist

• Implementation completed
• Peer reviewed
• Security review completed
• Ready for merge