fix(auth)!: the recovery phrase is proved by a key of its own

A key proof was a signature by the account's signing key, which every
signed-in device keeps in its keyring: whoever could read one, with a
code from its authenticator, set a new password without the password or
the phrase (the second reading's H1). The phrase now proves itself with
an Ed25519 key derived from its bits alone (halkyon_keys'
recoveryProofKeysOf, KDF context hlkrprf1, a sibling of the recovery
key), whose public half every setup, new phrase and account reset sends
and the register keeps (account_password.recoveryProofKey); the device
derives the secret half only while the phrase is typed. An account
without one (none exist outside development) proves no phrase until it
sets a new one; decoys check against a decoy proof key alike.

Migration (module, Learn's and Todo's servers, Learn's device schema):
ALTER TABLE "account_password" ADD COLUMN "recoveryProofKey" bytea;
nullable, so rows made before keep working and no data is touched.

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

@@ -20,6 +20,7 @@ part '20261019201023295-profiles-in-halkyon-base/migration.dart';
 part '20261019204112282-deletion-notices/migration.dart';
 part '20261020111943300-passwords/migration.dart';
 part '20261020125117584-sessions-ended/migration.dart';
+part '20261020185750538-recovery-proof-key/migration.dart';
 
 /// Migration registry for the client-side database.
 class MigrationRegistry {
@@ -42,6 +43,7 @@ class MigrationRegistry {
     _Migration20261019204112282_deletion_notices(),
     _Migration20261020111943300_passwords(),
     _Migration20261020125117584_sessions_ended(),
+    _Migration20261020185750538_recovery_proof_key(),
   ];
 
   /// List of all client-side database migration versions.