Turn a mail into task candidates, and into nothing else (#246)

The extraction half. internal/email.Extractor asks the resident Qwen3-1.7B,
under a GBNF grammar, what one message requires of him, and returns at most
three short candidates with an optional date.

Everything it can produce is a row in `tasks` with status "candidate",
written through the intake seam #130 built for exactly this (Source
"email:<mailbox>", Evidence = the subject line). No reminder, no fact, no
note, no calendar event. That bound is the design: a reminder FIRES, so a
1.7B misreading "встреча была в четверг" as a future appointment would wake
him up about it, whereas a wrong candidate is a line he dismisses in one
click. A due date the model read out of the mail is stored on the candidate,
where no scheduler reads it — the review page sorts by it. Relative wording
("до пятницы") is deliberately left in the text rather than resolved to a
date the model would get wrong.

The prompt is written against the two things a small model does here: it
summarises when asked to extract, and it invents an obligation out of a
polite closing line. Hence the demand for a verb phrase, and an explicit
empty array — most mail contains no task, and a model with no way to say
"nothing" says something.

Wiring: core owns extraction because llama-server lives in core's process,
so the reader hands messages over a new ipc.MethodIngestMail. It is a Server
hook (like StepUp/UnlockFn), not a CoreAPI method — not a store operation,
and no CoreAPI implementation should have to carry it. The hook stays nil
without an `email` config block or without a llama-server phraser, so the
method answers ErrUnknownMethod: off unless configured, twice over. There is
no keyword fallback on purpose — "the subject became a task" is a mailbox
rendered as a to-do list, not extraction.

Privacy: junk is refused before the model is called, mail text is never
search input, extraction errors carry byte counts rather than the reply, the
stored evidence is a truncated subject, and the log line names the mailbox
and the UID only.

cmd/mavend/main.go

@@ -320,6 +320,13 @@ func run(args []string) error {
 
 	srv.StepUp = func(ctx context.Context) error { return passkeySess.Assert(ctx, auth.Scope{}) }
 
+	// Mail ingestion (Vikunja #246): the hook stays nil unless an email block is
+	// configured and there is a llama-server to extract with, in which case
+	// ipc.MethodIngestMail reports ErrUnknownMethod.
+	if !locked {
+		wireMailIntake(srv, st, phr, cfg)
+	}
+
 	// WrapKeyFn — wraps the env key with a passkey credential public key and
 	// persists the wrapped blob. Only wired when the daemon has the key in
 	// memory (env key mode). Called by mavweb after passkey enrollment.
@@ -457,6 +464,7 @@ func run(args []string) error {
 			}
 			srv.SetAPI(newAPI)
 			srv.Check = (&auth.Gate{Enrollment: auth.NewFloorEnrollment(), Session: passkeySess}).Check
+			wireMailIntake(srv, st, phr, cfg)
 
 			// Start voice server.
 			if voiceW != nil {