docs(adr): private studysets (ADR 0019)

The developer's decisions of 21 Oct: private studysets ship with build 22;
a data key per private studyset and a personal key for tags and settings,
sealed to the account and kept in a synced table; files encrypted too; a
release's tag names and the copies counted come from the device; the shape,
size and rhythm of studying stay readable and are listed; existing
studysets get one offer; accounts without keys at the register wait for a
password. ADR 0002 and ADR 0017 point to it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

docs/adr/0002-no-end-to-end-encryption.md

@@ -2,6 +2,8 @@
 
 - **Status:** accepted
 - **Date:** 2026-09-29
+- **Amended by:** [ADR 0019](0019-private-studysets.md), which takes the way back below:
+  private studysets are end-to-end encrypted.
 - **Amends:** [ADR 0001](0001-offline-sync.md), whose context assumed that private and
   shared data would be end-to-end encrypted later.