feat(analytics): add explicit `source` param + 'system' default to capture/captureException

[PierreBrisorgueil]

Problem

Today analytics.capture() accepts an optional req (whose posthogContext.source is merged into event properties), and analyticsService.captureException(err, ctx) accepts {distinctId, requestId}. Neither has an explicit source parameter.

Consequence:

• Server-side emissions with no req (scrap_run_completed from worker callback, compute_consumed from historys, subscription_changed from Stripe webhook, cron tasks) end up with source = null in PostHog.
• Validated 2026-05-11: 100% of Vue events have null source (expected), but all 3 Node events from a manual scrap run also had null source — worker-callback and stripe-webhook emissions can't be attributed.

Spec

Extend capture({distinctId, event, properties, req, source}) and captureException(err, {distinctId, requestId, source, properties}) to accept an explicit source override. Precedence: explicit source > properties.source > req.posthogContext.source > 'system' default.

Acceptance criteria

• capture() adds optional source param to JSDoc + destructure
• captureException() accepts ctx.source and ctx.properties
• Default 'system' baked into the merged defaults of both helpers
• Unit tests cover the 4-tier precedence
• Coverage thresholds unchanged
• README convention table updated

Follow-up after merge:

• Trawl Node downstream audit (separate issue)
• Winston transport (feat(observability): forward Winston error-level logs to PostHog Error Tracking via custom transport #3651) uses new captureException ctx.properties