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

docker-compose.yml

@@ -113,6 +113,31 @@ services:
       - sockets:/run/maven
       # - ./deploy/zenmoney.token:/run/secrets/zenmoney.token:ro
 
+  # The mail reader (Vikunja #246) is OFF and commented out: it needs an IMAP
+  # account, and there is none on this box. mavmaild reads the password from a
+  # FILE so it never appears in `ps`, in this file, or in shell history — the
+  # same rule mavpoll follows for the zenmoney token. Core never sees the
+  # password: the reader hands core message text on one IPC method, and core
+  # writes what the model extracts as task CANDIDATES he reviews on /tasks.
+  # Nothing here can create a reminder, so a misread mail cannot fire.
+  #
+  # To enable: write the password to deploy/imap.password (0600, gitignored),
+  # add an "email": {} block to deploy/mavend.json, and uncomment this service.
+  # mavmaild:
+  #   <<: *image
+  #   command: ["mavmaild", "-socket", "/run/maven/mavend.sock",
+  #             "-imap", "imap.example.org:993",
+  #             "-user", "kami@example.org",
+  #             "-password-file", "/run/secrets/imap.password",
+  #             "-mailbox", "INBOX",
+  #             "-interval", "15m",
+  #             "-state", "/var/lib/maven/mail-seen.json"]
+  #   depends_on: [mavend]
+  #   volumes:
+  #     - sockets:/run/maven
+  #     - dbdata:/var/lib/maven
+  #     - ./deploy/imap.password:/run/secrets/imap.password:ro
+
 volumes:
   dbdata:
   sockets: