fix(auth)!: a first password takes a fresh proof of the account's way in

The fourth reading's first finding: password.set took a session alone,
so whoever took one made a GitHub-only account, or an address account
without a password, theirs for good (their address or password, their
authenticator; GitHub then refused the owner).

- An account with an address proves it anew: a code mailed to that
  address for this (EmailCodePurpose.firstPassword, its own mail), never
  the session alone, a code asked for anything else, or another address.
- An account that signs in with GitHub alone does it within 10 minutes of
  a GitHub sign-in (the session's method and creation time, where the
  session lives; members tell the register), refused signInAgain before
  any code is used; the address it adds is proved as before.
- Anonymous people still become identified with a code to the address
  they add.

The app mails the code to the person's own address (asking for it where
the device doesn't know it), offers a password right after a GitHub
sign-in (D2's "at their next sign-in") before the person's app, and asks
for a GitHub sign-in anew when the instance refuses an older one, keeping
the code typed.

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

packages/halkyon_base_server/test/integration/support/fixtures.dart

@@ -55,6 +55,7 @@ Future<UuidValue> signInNewDevice(
   Session session,
   UuidValue person, {
   Set<Scope> scopes = const {},
+  String method = 'test',
 }) async {
   Future<Set<UuidValue>> sessionIds() async => {
     for (final serverSideSession in await ServerSideSession.db.find(
@@ -68,7 +69,7 @@ Future<UuidValue> signInNewDevice(
   await AuthServices.instance.tokenManager.createToken(
     session,
     authUserId: person,
-    method: 'test',
+    method: method,
     scopes: scopes,
   );
   return (await sessionIds()).difference(before).single;