feat(base,app,keys): keys of shared spaces, sealed to each member

Step 3 of ADR 0044 (D3, D18, D19). A shared space's data keys are
sealed on a device to each member's public keys; the instance keeps
what devices sealed and never opens it.

- halkyon_keys: the safety code of two accounts' public keys
  (`safetyCode`), 60 digits, the same on both sides.
- halkyon_base: the register hands out accounts' public keys
  (`public-keys-of`, local and remote); the table `shared_space_key`
  and the endpoint `sharedSpaceKeys` (memberKeys, keysNeeded, seal,
  mine). Only the Owner and Editors seal, the first key sealed to a
  member stays, and a member's keys go with their membership. Every
  host's migration creates the table.
- halkyon_app: `ContentCipher` seals a key to another account and
  opens one sealed by another; `SharedKeys` shares a space's key with
  its members, seals keys to newcomers and opens the person's own,
  holding each member to the public keys `KeyPins` pinned on first
  sight (`key_pins.json` in the person's folder), a change refused
  until trusted anew.

Learn takes them into its `Privacy` in step 4.

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/train/halkyon_train_client/lib/migrations/migration_registry.dart

@@ -14,6 +14,7 @@ part '20261102113104496-strength-plans/migration.dart';
 part '20261103204323683-body-weights/migration.dart';
 part '20261110225636220-file-store/migration.dart';
 part '20261111091721828-shared-spaces/migration.dart';
+part '20261111100125761-shared-space-keys/migration.dart';
 
 /// Migration registry for the client-side database.
 class MigrationRegistry {
@@ -30,6 +31,7 @@ class MigrationRegistry {
     _Migration20261103204323683_body_weights(),
     _Migration20261110225636220_file_store(),
     _Migration20261111091721828_shared_spaces(),
+    _Migration20261111100125761_shared_space_keys(),
   ];
 
   /// List of all client-side database migration versions.