feat(auth): a session ended for its wrong proofs warns its owner

The tenth wrong owner proof in a row ends a session (the third
reading); someone holding that session tried the password or its
codes. The register now keeps the moment with the account's password
(`account_password.sessionEndedForProofsAt`): the account's sign-in
ways and its next password sign-in carry it, until the owner dismisses
it (`password.dismissSessionEndedWarning`, their session alone: it
hides a warning and secures nothing) or sets a new password. A member
ending its own session tells the register
(`noteSessionEndedForProofs`); a register out of reach leaves the
owner unwarned, the session ended all the same.

Migration (module, Learn's and Todo's servers, Learn's device schema):
ALTER TABLE "account_password" ADD COLUMN "sessionEndedForProofsAt"
timestamp without time zone; nullable, so every row keeps its values
and has no warning.

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

@@ -24,6 +24,7 @@ part '20261020185750538-recovery-proof-key/migration.dart';
 part '20261020194228286-second-factor-waits/migration.dart';
 part '20261020204619104-session-code-tries/migration.dart';
 part '20261020211224692-email-code-purpose/migration.dart';
+part '20261021100422048-session-ended-warning/migration.dart';
 
 /// Migration registry for the client-side database.
 class MigrationRegistry {
@@ -50,6 +51,7 @@ class MigrationRegistry {
     _Migration20261020194228286_second_factor_waits(),
     _Migration20261020204619104_session_code_tries(),
     _Migration20261020211224692_email_code_purpose(),
+    _Migration20261021100422048_session_ended_warning(),
   ];
 
   /// List of all client-side database migration versions.