[Phase 7] Set up Production Observability (Sentry, PostHog, Better Stack)

[VenkataAnilKumar]

Goal

Establish baseline production observability across web/api/worker/postprocess.

Scope

• Sentry error capture and release tagging
• PostHog product analytics events for core funnel
• Better Stack log aggregation and alerting

Acceptance Criteria

• Runtime errors are searchable by service and release
• Core product funnel events are visible in PostHog
• On-call alerts trigger for critical failures