Sentry: sync-session transaction + bg/fg breadcrumbs (usage tier)

[gmaclennan]

Part of #74. Depends on #75/#77 (both landed via PR #111). Re-scoped after #111: the recordUsage.screen/feature helpers are dropped from scope — front-end feature/screen usage telemetry is handled by the host app via PostHog, not Sentry (see docs/sentry-integration.md §9.2). Sentry stays focused on bug reporting and performance.

Spec: docs/sentry-integration-plan.md Phase 5 (what remains usage-tier) + §11.4.

Deliverables

• A comapeo.sync.session transaction covering each sync session, with child spans for discover / handshake / replicate. Carries only bucketed peer count, bucketed bytes, and outcome — no peer identities, no raw project IDs (§11.3)
• Wire the already-shipped metrics.syncSession(outcome, ms, peersBucket, bytesBucket) emitter (backend/lib/metrics.js) from the same lifecycle plumbing — the helper and its peers/bytes bucket functions exist but currently have no call site, so comapeo.sync.session.duration_ms and the bucket counters never emit
• Background/foreground breadcrumbs from React Native AppState changes, so errors can be read as "fired 3 seconds after the app was backgrounded"
• Remove the "not yet wired" markers from the sync rows of the §9.2 tier table in docs/sentry-integration.md

Tests

• Toggle on ⇒ usage-gated captures present; toggle off ⇒ absent (the duration metric stays diagnostic-tier)
• Sync-session transaction carries only the allowed bucketed values

[gmaclennan]

Re-scoped: recordUsage.screen/feature dropped — front-end usage telemetry goes to PostHog, not Sentry. Added the metrics.syncSession emitter wiring (helper shipped unwired in PR #111). Original body is in the edit history.