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>

nextcloud_mcp_server/server/__init__.py

@@ -7,6 +7,7 @@ from .collectives import configure_collectives_tools
 from .contacts import configure_contacts_tools
 from .cookbook import configure_cookbook_tools
 from .deck import configure_deck_tools
+from .mail import configure_mail_tools
 from .news import configure_news_tools
 from .notes import configure_notes_tools
 from .semantic import configure_semantic_tools
@@ -30,6 +31,7 @@ AVAILABLE_APPS: dict[str, Callable[[FastMCP], None]] = {
     "cookbook": configure_cookbook_tools,
     "deck": configure_deck_tools,
     "news": configure_news_tools,
+    "mail": configure_mail_tools,
     "talk": configure_talk_tools,
 }
 
@@ -40,6 +42,7 @@ __all__ = [
     "configure_contacts_tools",
     "configure_cookbook_tools",
     "configure_deck_tools",
+    "configure_mail_tools",
     "configure_news_tools",
     "configure_notes_tools",
     "configure_semantic_tools",