feat(base)!: no anonymous accounts on the server (ADR 0020, D25)

The server's half of D25: every account has a way in, and the anonymous
accounts left from before go with the daily pruning.

- The module signs nobody in anonymously: `anonymousIdp` and
  `gitHubUpgrade` are gone, and so is linking GitHub at the register.
- A password is set only by someone whose address is theirs (a
  `firstPassword` code) or who signs in with GitHub alone (a fresh
  sign-in and a `signUp` code); an anonymous account is refused
  `wrongCredentials` before any code is used.
- SimpleLogin refuses to link onto a caller without `identified`
  (`wrongCaller`), the member's endpoint too, so nothing turns an
  anonymous account into one with a way in.
- `pruneAnonymousPeople` deletes every anonymous account left, however
  new, never one identified. Learn and Todo schedule it as
  `prune-accounts` and cancel the old `prune-abandoned-anonymous-people`
  entry once.
- Learn: someone without `identified` keeps no files;
  `FILE_LIMIT_ANONYMOUS_MB` is gone. `minimumAppBuild` is 23, as builds
  up to 22 start with an anonymous account.
- Learn's sync tests make their person and session by SQL in the
  development database, as Serverpod stores sessions.
- Docs, store listings and ADRs 0010, 0014, 0016, 0017 and 0019 amended.

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/README.md

@@ -12,8 +12,11 @@ instance are published, so anyone can see how an instance works and run their ow
   card content is stored as append-only revisions, and a side changed differently on two
   devices is shown as a conflict until the person picks a version
   ([ADR 0001](../../docs/adr/0001-offline-sync.md), [ADR 0003](../../docs/adr/0003-card-revisions.md)).
-- **No sign-up needed.** The app starts with an anonymous account; signing in with GitHub
-  later keeps everything. A GitHub sign-in stores only the numeric GitHub ID.
+- **Every account has a way in**: SimpleLogin, whose ID alone the instance keeps, or an
+  email address with a password and an authenticator, the password never leaving the
+  device ([ADR 0020](../../docs/adr/0020-sign-in-with-simplelogin.md),
+  [ADR 0017](../../docs/adr/0017-password-totp-and-keys.md)). GitHub still signs in
+  those who have it, until the instance ends it.
 - **No end-to-end encryption.** Content is readable by the operator of the instance, as on
   Mastodon; whoever trusts no operator can run their own
   ([ADR 0002](../../docs/adr/0002-no-end-to-end-encryption.md)).