feat(mail): read and index Nextcloud Mail via the Mail OCS API

Add read-only support for the Nextcloud Mail app, plus semantic indexing
of mail messages. The MCP server never speaks IMAP/POP3 itself: it calls
the Mail app's CSRF-free OCS API (/ocs/v2.php/apps/mail/api/...) with the
existing Basic-Auth app-password flow and an OCS-APIRequest header, and the
Mail app handles IMAP server-side.

- client/mail.py: MailClient (accounts, mailboxes, messages, message,
  attachment), OCS-envelope aware.
- models/mail.py: Pydantic models with the API's camelCase aliases.
- server/mail.py: 5 read-only MCP tools (mail.read scope), registered in
  AVAILABLE_APPS.
- Vector pipeline: new "mail_message" doc_type wired into scanner
  (scan_mail_messages, newest-N per mailbox), processor (body -> markdown
  embedding), per-id verifier, and context expansion.
- Tests: client API, model round-trips, verifier behavior; consent-backstop
  test now derives its allowed set from INDEXED_DOC_TYPES.
- README + semantic-search docstrings updated.

Requires Mail 5.x / Nextcloud 32+ and a mail account configured in the
Mail app. Follow-up: astrolabe must advertise "mail_message" in its
enabled_doc_types capability for search under admin doc_type restriction.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/unit/vector/test_scanner_consent_backstop.py

@@ -104,7 +104,9 @@ async def test_noop_when_allowed_is_none(monkeypatch):
 
 async def test_noop_when_all_text_types_allowed(monkeypatch):
     send = AsyncMock()
-    allowed = frozenset({"note", "news_item", "deck_card", "file"})
+    # Derive from INDEXED_DOC_TYPES so a newly-indexed text type doesn't make
+    # this "all allowed" set silently incomplete (and trip the backstop).
+    allowed = frozenset(scanner_module.INDEXED_DOC_TYPES)
     queued = await _enqueue_deletes_for_disabled_types(
         "alice", _producer(send), allowed, 1
     )