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/search/context.py

@@ -822,6 +822,48 @@ async def _fetch_document_text(
             if card.description:
                 content_parts.append(card.description)
             return "\n\n".join(content_parts)
+        elif doc_type == "mail_message":
+            # Mail message IDs are positive ASCII integers (MySQL AUTO_INCREMENT).
+            if not is_valid_nextcloud_doc_id(doc_id):
+                logger.warning(
+                    "Expected numeric mail_message doc_id, got %r — skipping document fetch",
+                    doc_id,
+                )
+                return None
+            # Reconstruct full content as indexed by the processor (subject +
+            # From + To + blank line + body) so chunk offsets align. Keep this in
+            # sync with the mail_message branch in vector/processor.py.
+            message = await nc_client.mail.get_message(int(doc_id))
+
+            def _format_addresses(addrs: list[dict] | None) -> str:
+                parts = []
+                for addr in addrs or []:
+                    label = addr.get("label")
+                    email = addr.get("email")
+                    if label and email and label != email:
+                        parts.append(f"{label} <{email}>")
+                    elif email:
+                        parts.append(email)
+                    elif label:
+                        parts.append(label)
+                return ", ".join(parts)
+
+            subject = message.get("subject") or ""
+            from_str = _format_addresses(message.get("from"))
+            to_str = _format_addresses(message.get("to"))
+            raw_body = message.get("body") or ""
+            body_text = (
+                html_to_markdown(raw_body) if message.get("hasHtmlBody") else raw_body
+            )
+
+            content_parts = [subject]
+            if from_str:
+                content_parts.append(f"From: {from_str}")
+            if to_str:
+                content_parts.append(f"To: {to_str}")
+            content_parts.append("")  # Blank line
+            content_parts.append(body_text)
+            return "\n".join(content_parts)
         else:
             logger.warning("Unsupported doc_type for context expansion: %s", doc_type)
             return None