docs(compliance): record the deletion trigger for client-side storage (#1246)

Answers the ISO 27001 A.8.10 finding from the compliance swarm on #1242,
which read the storage inventory as saying the two localStorage keys
persist forever with no deletion mechanism. Half of that was our
omission: the inventory never said what logout does.

ph_conv_<token> IS deleted on logout. posthog.reset() resets the
conversations manager, which removes that single known key, and
resetAnalyticsIdentity() already runs in both logout handlers before
signOut(). That is what stops a shared device carrying one user's
support-ticket session into the next user's. Recorded in
lib/analytics/reset.ts too, because it now makes that call load-bearing
for a cross-user concern rather than mere tidiness. Verified by reading
the SDK, not by executing a logout, and the docs say so.

seenSurvey_<id> genuinely has no deletion trigger and cannot have one:
no PostHog bundle enumerates localStorage (zero occurrences of
localStorage.key( or Object.keys(localStorage) across module.js,
surveys.js and conversations.js), so nothing can discover the keys to
remove them. Stated as the accepted retention position rather than left
silent: the value is "true" under an opaque survey id with no personal
data, and clearing it on logout would re-prompt every survey to the next
person on the device and produce false survey responses.

Adds the review date the A.5.9 remediation asked for, and reframes the
review trigger as a pre-launch step, since Support was caught post-hoc
and left the privacy page inaccurate in the interval.

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

DECISIONS.md

@@ -619,3 +619,4 @@ One line per decision: `[YYYY-MM-DD] <decision>: <why>`. Appended by agents and
 [2026-07-27] Dropbox web links point at /home/Apps unless DROPBOX_APP_FOLDER_NAME is set: app-folder scoped calls cannot discover where the app folder sits in the user's account, and a link into the wrong folder reads as a lost backup, so the app-name deep link is opt-in rather than guessed.
 [2026-07-27] performSync keeps `provider` optional, defaulting to Google Drive: it preserves the pre-Dropbox call shape (and its test suite) as the documented legacy path while all three production call sites pass a provider explicitly.
 [2026-07-27] Privacy page corrected after verifying prod: PostHog's conversations module writes ph_conv_<token> = {"widgetSessionId": uuid} to localStorage at PAGE LOAD, not when support is contacted, bypassing persistence: 'memory' exactly like surveys' seenSurvey_ flags. The page previously claimed enkäter were the only local storage, which was wrong the moment Support was enabled. Both values are functional UI state without personuppgifter (do-not-ask-again, and find-my-own-ticket), so the no-consent-banner position holds, but "inga kakor eller annan lagring" did not and is now stated accurately. Worth remembering that enabling any new PostHog product can silently add device storage: check localStorage in prod after each one rather than trusting the persistence setting.
+[2026-07-27] Documented the two client-side storage keys as having DIFFERENT lifecycles, after the compliance swarm flagged ISO 27001 A.8.10 (no deletion mechanism) on the storage inventory. ph_conv_<token> IS deleted on logout: posthog.reset() calls the conversations manager's own reset, which removes that single known key, and resetAnalyticsIdentity() already runs in both logout handlers. That is what stops a shared device carrying one user's support-ticket session into the next user's session. seenSurvey_<id> genuinely has no deletion trigger, and cannot have one from the SDK: no PostHog bundle enumerates localStorage (zero occurrences of localStorage.key( or Object.keys(localStorage) across module.js, surveys.js, conversations.js), so nothing can discover the keys to remove them. Accepted as the retention position rather than an oversight: the value is "true" under an opaque survey id with no personal data, and clearing it on logout would re-prompt every survey to the next person on the device and produce false survey responses.