fix(telemetry): data-quality pass (opt-in noise, onboarding split, file_count, OIDC) (#614)

Five fixes to the PostHog event stream, from an audit of what we actually collect versus what's flowing in. Each one is test-first.

## What changed

**Silenced the `$opt_in` noise.** `initAnalytics` called `opt_in_capturing()` on every page load to clear a stale opt-out flag, and posthog-js emits an `$opt_in` event on every call. That was 10k+ events a month (up to 55 per user) carrying no signal: analytics is on by default with an admin opt-out, so there is no per-user consent to record. Both call sites now pass `captureEventName: false`.

**Split the onboarding survey out of `feedback_submitted`.** The onboarding usage survey rode the same event as real feedback, so about 93% of "feedback" was actually onboarding profiling. It now emits `onboarding_survey_submitted`, so feedback metrics mean feedback again.

**Set `pipeline_executed.file_count`.** It was declared in the properties interface but never populated. A pure `pipelineExecutedProps` helper now derives it (batch size for a batch run, else 1) and is shared by the success and failure paths, which also drops a duplicated payload.

**Tracked OIDC login failures.** All six OIDC callback failure branches bumped the Prometheus counter and wrote an audit log but never emitted `auth_login_failed`. A `recordOidcFailure` helper mirrors the password path.

**Added `TELEMETRY.md`.** A contributor-facing event dictionary: every event, its properties, where it fires, and the privacy invariants, with the allowlists as source of truth. A drift test fails if any `ANALYTICS_EVENTS` value goes undocumented.

I left the published telemetry guide (`apps/docs/guide/telemetry.md`) alone. It is high-level and still accurate, and editing it would pull in the 21-locale stale-gate for no gain.

## Verification

- Unit (63 tests): `analytics-events`, `telemetry-doc-drift`, `api/analytics`, `web/analytics`, `worker.behavior`
- Integration (41 tests): `oidc-auth`, `feedback`
- Full typecheck across all 9 workspaces
- Biome clean on the changed files

All green locally.

apps/web/src/lib/analytics.ts

@@ -89,8 +89,11 @@ export async function initAnalytics(config: AnalyticsConfig): Promise<void> {
     // Clear any persisted opt-out from a previous disabled period. opt_out_capturing()
     // writes a localStorage flag that survives reloads, so without this a browser that
     // once opted out would stay silent even after the instance re-enables analytics.
+    // captureEventName: false suppresses posthog-js's default $opt_in event, which
+    // otherwise fires once per page load here (pure noise: analytics is on by
+    // default with an admin opt-out, so there is no per-user consent to record).
     try {
-      posthog.opt_in_capturing();
+      posthog.opt_in_capturing({ captureEventName: false });
     } catch {
       // ignore
     }
@@ -194,7 +197,9 @@ export function optOut(): void {
 export function optIn(): void {
   enabled = true;
   try {
-    posthog?.opt_in_capturing();
+    // captureEventName: false: resuming capture is not a per-user consent signal
+    // in this product, so don't emit a noisy $opt_in event (see initAnalytics).
+    posthog?.opt_in_capturing({ captureEventName: false });
   } catch {
     // ignore
   }