_extract_body previously discarded real HTML content, replacing it with
a placeholder whenever a multipart email had no text/plain part. Now
falls back to the raw text/html payload before giving up, only using
the placeholder when a message has no text-bearing part at all.
poll_mailboxes also had no error isolation: an unhandled IMAP exception
from any one mailbox would stop the discord.ext.tasks.Loop permanently,
silently killing ingestion for all three mailboxes. Each mailbox's poll
is now wrapped in try/except so one failure doesn't block the others.
