feat(privacy): the instance's side of private studysets (ADR 0019)

- `content_key`, a synced table in the person's space, keeps their data keys
  sealed to their account (a private studyset's, their personal one);
  `learning_path_answer` gains `keyId` and `nonce` for answers in private
  studysets. Additive only, on the server and the device.
- Publishing refuses a private studyset, and a tree holding a private row
  (`PublishRefusal.private`); a private studyset elsewhere no longer breaks
  publishing a readable one. Encrypted tags are named as the device names
  them (`tagNames`): a name for a tag that isn't the person's, or an
  encrypted tag of the release without one, refuses it
  (`tagNamesMismatch`); a readable tag keeps its own name.
- The connector leaves private studysets out and says how many there are.
- The learner count reads no payload any more: Count me is kept by the
  instance (`counted_learner`) and the device reports which publications
  its copies come from (`learning_copy`, accepted only while counted, each
  report replacing the last); only the reporter's own studysets count.
  Deleting a person deletes both.
- An upload says whether its file is encrypted: only a readable one over
  20 MB is checked to be a PDF, and an encrypted one may be 4 KiB larger.

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

apps/learn/halkyon_learn_client/lib/migrations/migration_registry.dart

@@ -27,6 +27,7 @@ part '20261020211224692-email-code-purpose/migration.dart';
 part '20261021100422048-session-ended-warning/migration.dart';
 part '20261021110159867-second-factor-paths/migration.dart';
 part '20261021134803749-reset-hold-outs/migration.dart';
+part '20261021184601197-private-studysets/migration.dart';
 
 /// Migration registry for the client-side database.
 class MigrationRegistry {
@@ -56,6 +57,7 @@ class MigrationRegistry {
     _Migration20261021100422048_session_ended_warning(),
     _Migration20261021110159867_second_factor_paths(),
     _Migration20261021134803749_reset_hold_outs(),
+    _Migration20261021184601197_private_studysets(),
   ];
 
   /// List of all client-side database migration versions.