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>

deploy/.env.example

@@ -146,14 +146,12 @@ DATA_LOCATION=
 BACKUP_LOCATION=
 
 # How much room people's images and sounds may take on this instance, in MiB. Empty: the
-# defaults, 100 for an anonymous person (anonymous accounts are meant to become identified
-# ones), 500 for one signed in with GitHub, and 15000 for everyone together (0: no limit
-# for everyone together). Every file is in the database, so also in each of the 3 dumps
+# defaults, 500 for a person and 15000 for everyone together (0: no limit for everyone
+# together). Every file is in the database, so also in each of the 3 dumps
 # this server keeps (and a fourth while a deploy writes its own): files take five times
 # their size on the disk. One person may get more than the default; as the operator,
 # in deploy/ (their id is their account's, "authUserId"):
 #   docker compose exec postgres psql -U halkyon_learn -d halkyon_learn -c "INSERT INTO
 #     file_allowance (\"authUserId\", \"maxBytes\", note) VALUES ('<id>', 2147483648, 'asked by mail')"
-FILE_LIMIT_ANONYMOUS_MB=
 FILE_LIMIT_IDENTIFIED_MB=
 FILE_LIMIT_INSTANCE_MB=