feat(support): open PostHog tickets from the existing support dialog (#1239)

Enables PostHog Support through the Direct API (posthog.conversations),
restoring the second channel Recapt used to provide, but as a real ticket
linked to the person and their session replay instead of a black hole.

The in-app WIDGET stays off on purpose. It is a third-party floating chat
bubble, which is exactly what Recapt was: it would sit next to the
Assistenten FAB (which already has a hide_assistant_fab preference
because users wanted it gone), cannot follow the locked design system,
and its copy is not ours to keep Swedish. The conversations API gives the
same tickets from components/ui/support-link.tsx, which is already
on-design, Swedish and reachable from 8 surfaces.

A ticket is explicitly NOT treated as delivery. submitFeedback returns ok
only when the Resend email actually went out, even if the ticket opened.
Recapt's precise failure mode was reporting success on its own channel
while /api/support/contact was dead, and nobody is watching PostHog at
02:00. Tests pin that: ticket-only is ok:false.

Identity verification uses posthog.setIdentity(distinctId, hash) at
runtime rather than the identity_distinct_id/identity_hash init options
PostHog's settings page documents. init runs from
instrumentation-client.ts app-wide, before the user is known and
including logged-out pages, and PostHog fixes init values for the
session. setIdentity is a real method on the SDK (verified typed in
posthog-js 1.407.3), so the hash applies from AnalyticsIdentify once the
dashboard layout knows who the user is. Without the key it is skipped and
tickets fall back to browser-scoped with email recovery, which is the
normal state off hosted.

POSTHOG_SECRET_API_KEY is server-only, no NEXT_PUBLIC_ prefix: it signs
identity hashes AND authenticates external API requests, so unlike the
phc_ project token it is a real credential. Only the derived per-user
HMAC crosses to the browser.

Compliance: support free text is declared as its own data category
(user.content.support) in .compliance/ropa.yaml and named on the privacy
page. Analytics events still carry no message body (the breadcrumb sends
only the subject); a ticket carries what the user wrote, because that is
the point. Keeping the purposes separate is what stops the privacy page
drifting the way the Recapt row did.

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

.env.example

@@ -65,6 +65,14 @@ CRON_SECRET=generate-a-random-secret
 # NEXT_PUBLIC_POSTHOG_HOST is only used by the server-side SDK.
 # NEXT_PUBLIC_POSTHOG_PROJECT_TOKEN=
 # NEXT_PUBLIC_POSTHOG_HOST=https://eu.i.posthog.com
+#
+# PostHog Support identity verification. A REAL SECRET (it also authenticates
+# external API requests), so no NEXT_PUBLIC_ prefix: it must never reach the
+# client bundle. Only the derived per-user HMAC crosses to the browser
+# (lib/analytics/identity-hash.ts). Unset means support tickets are scoped to
+# one browser session and users recover them by email link, which is the
+# normal state for local dev, CI and self-hosted.
+# POSTHOG_SECRET_API_KEY=
 
 # ── Optional: error tracking / observability ──────────────
 # The app routes every error-level log line, and anything flagged