Sentry: review-pipeline tracing + per-installation tagging

[JSONbored]

Part of #998.

Context

Trace the review pipeline end-to-end and tag events per installation so errors/perf can be sliced by tenant (closes the per-tenant observability gap).

Deliverables

• Spans across webhook → queue → dual-AI review → gate decision → comment/merge post, with timing.
• Tag events with repo, hashed installation_id, operation/agent, and decision outcome (no raw secrets/scores).
• Configurable sample rate with a sensible default for hosted load.
• Surface AI-provider / fallback / quota events as breadcrumbs/spans.

Requirements / acceptance

• A single review shows one connected trace covering the pipeline stages.
• Errors and slow traces are filterable by installation in Sentry.