perf(mail): batch verify-on-read; test build_mail_content; addr-recall

Address PR #935 round-3 review:

- search/verification.py: rewrite _verify_mail_messages to batch by mailbox.
  get_message triggers a server-side IMAP body fetch, so per-result verify
  issued one IMAP FETCH per hit; now it calls the DB-cached list_messages once
  per mailbox (mailbox_id comes from the Qdrant payload via result.metadata)
  and intersects — O(unique mailboxes) light calls instead of O(results) IMAP.
- vector/mail_content.py: include Cc/Bcc in the indexed text so recipient
  queries match; move MAIL_SCAN_MAX_PER_MAILBOX here (shared by scanner index
  window + verifier presence window) with a note that it equals the Mail OCS
  per-request max (100), so it's a fixed constant not a config knob.
- client/mail.py: clamp list_messages limit to 1..100 at the client layer.
- tests: add test_mail_content.py (exact-layout contract for build_mail_content);
  rewrite the mail verifier tests for the batch-per-mailbox shape.

Left as-is: ValidationError isn't caught in the list-endpoint tools — consistent
with nc_notes_*/nc_deck_* and not a regression.

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

nextcloud_mcp_server/client/mail.py

@@ -142,7 +142,10 @@ class MailClient(BaseNextcloudClient):
             List of message summary objects (keys include databaseId, subject,
             from, to, dateInt, flags, previewText, mailboxId).
         """
-        params: dict[str, Any] = {"limit": limit}
+        # The Mail OCS API clamps limit to 1..100 server-side; do it here too so
+        # the contract is enforced at the client layer with a predictable value
+        # (a limit of 0 would otherwise collapse to 1 server-side).
+        params: dict[str, Any] = {"limit": min(max(1, limit), 100)}
         if cursor is not None:
             params["cursor"] = cursor
         if search_filter is not None: