Rename the identifiers that carried the upstream name

Everything clients, users and operators meet now carries the fork's name,
with no aliases (SPEC.md §2.4, changed here from "protocol identifiers
stay"):

- JMAP: upstream's registry capability is urn:inbuxa:jmap:registry, beside
  the fork's own urn:inbuxa:jmap.
- WebDAV lock and sync tokens are urn:inbuxa:dav*; clients resync once.
- Sieve: vnd.inbuxa.while and vnd.inbuxa.expressions. sieve-rs spells these
  into its compiler, so it's vendored (vendor/sieve-rs, 0.7.3) and patched in;
  a unit test fails if Cargo.lock ever moves past the vendored copy. The
  trusted runtime now names itself too, rather than answering sieve-rs's
  default.
- The web interface's OAuth client is inbuxa-webui. On every start the old
  stalwart-webui client is removed and any application naming it is moved
  over.
- The spam filter's blobs are INBUXA_SPAM_*; every start moves any left
  under the old keys, so a trained model survives.
- SQL stores and log files default to inbuxa, in the code and in the
  schema served to the admin (checksum regenerated).
- Settings are INBUXA_* only. A STALWART_* variable that's set where its
  INBUXA_* one isn't stops the server at startup, naming it.
- The version-upgrade messages link docs.inbuxa.org's migration page, and
  the OpenAPI description, smtp crate metadata and web-push test fixtures
  lose the name.

Kept on purpose, allowlisted with reasons: the OAuth key-derivation
contexts (renaming them would end every session and invalidate every
sealed client id) and the hashed application prefix.

Also fixes a latent start-up failure: ensure_client updated an existing
first-party client with a revision of 0, which the registry's assertion
never matches, so adding a redirect URI or changing the webmail secret
failed start-up. And the principal session test now expects
legacyProtocols (C-1, added 2026-09-21), which it had missed.

Tested: the server builds without warnings; common's 106 unit tests,
including the vendoring check; a new integration test for the two
start-up migrations; and the webdav, jmap, imap and SMTP Sieve suites.

docs/spec/contract.md

@@ -106,8 +106,9 @@ Each has an ID, and tests name the IDs they check.
     ihasmail-inbuxa server, authorization code with PKCE S256, redirect URI
     `{webmailUrl}/api/auth/callback`.
   INBUXA Admin's `<meta name="oauth-client-id">` is set to `inbuxa-admin`.
-  Until then it keeps upstream's `stalwart-webui`, which only works while
-  registration isn't required.
+  Served by the server itself it uses the web interface's client,
+  `inbuxa-webui` (upstream's `stalwart-webui` until 2026-09-22, retired on
+  start since).
 
   **Built (interim), 2026-09-18.** C-5's defaults are in the server, and
   `crates/common/src/manager/first_party.rs` registers the clients on every
@@ -116,7 +117,7 @@ Each has an ID, and tests name the IDs they check.
   `INBUXA_WEBMAIL_CLIENT_SECRET` (the webmail client is registered only when
   both of its variables are set). A web interface the server serves itself
   (none on a new install since SPEC.md §5.3; possible on one upgraded from
-  Stalwart) is registered too, as its application's OAuth client id or `stalwart-webui`, at the
+  Stalwart) is registered too, as its application's OAuth client id or `inbuxa-webui`, at the
   server's public URL. A missing client is created. An existing one gains any
   redirect URI it lacks, and the webmail client gets the configured secret.
   Nothing an operator added is removed. Bootstrap and recovery mode skip this: