Add mavmaild, the read-only IMAP poller that feeds mail intake (#246)

The extraction seam landed on the previous branch but nothing fed it. This
adds the daemon that does: every interval it opens one mailbox read-only
(EXAMINE + BODY.PEEK, so reading leaves no \Seen behind), fetches the UIDs
it has not handed over yet, and posts each message to core over
ingest_mail. Core runs the model and writes task candidates; this daemon
writes nothing and cannot create a reminder.

It is a separate daemon because of the credential. mavpoll set the
precedent with the zenmoney token (#125): the module talking to the third
party holds the secret, reads it from a file so it never lands in argv, in
docker-compose.yml or in shell history, and core never sees it. There is
deliberately no -password flag, and a test asserts that.

Off unless configured at both ends: without -password-file the daemon
refuses to start, and if core has no email block the first ingest returns
ErrUnknownMethod, which disables the reader instead of hammering a socket
that will keep refusing. A seen-UID state file (0600, atomic write) keeps a
restart from re-extracting the whole lookback window; correctness does not
depend on it, since capture dedupes on normalised text. Logs are counts and
UIDs — no subject, sender or body.

Verified with an in-process IMAP server and a fake core: bulk mail is
filtered before core is asked, seen UIDs are not re-fetched, a failed
ingest is retried next poll, ErrUnknownMethod stops at the first message,
and state survives a restart. The live half is untested by design — no IMAP
credential exists on this box; setup is written up as QA steps.

Vikunja #246

.gitignore

@@ -7,6 +7,7 @@
 /mavpoll
 /mavcaldav
 /mavwaked
+/mavmaild
 
 # Certs (private keys, don't commit)
 certs/
@@ -36,6 +37,8 @@ deploy/db_key.env
 deploy/telegram.env
 # zenmoney API token, read by mavpoll (never in argv, never committed)
 deploy/zenmoney.token
+# IMAP password, read by mavmaild (never in argv, never committed)
+deploy/imap.password
 
 # Temp files
 /tmp/