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.

internal/ipc/api.go

@@ -138,6 +138,44 @@ type CaptureTaskResp struct {
 	Created bool  `json:"created"`
 }
 
+// IngestMailReq — one message a mail reader has fetched, handed to core for
+// extraction (Vikunja #246).
+//
+// The mail reader (cmd/mavmaild) holds the IMAP credential and core never sees
+// it, the same split mavpoll uses for the zenmoney token. What crosses this
+// boundary is only the message text, because extraction runs on the resident
+// model and llama-server lives inside core's process.
+//
+// Body is already plaintext and truncated by internal/email; core does not
+// re-parse MIME and never stores the body. Junk means the reader's header
+// filter already classified the message as bulk — core is told rather than
+// asked, so a junk message can be counted without a model call.
+//
+// This method is available only when core has an email block configured AND a
+// llama-server phraser; otherwise it answers ErrUnknownMethod, which is what
+// "off unless configured" looks like at the wire.
+type IngestMailReq struct {
+	Mailbox string `json:"mailbox"`
+	UID     uint32 `json:"uid"`
+	From    string `json:"from,omitempty"`
+	Subject string `json:"subject,omitempty"`
+	Date    string `json:"date,omitempty"`
+	Body    string `json:"body,omitempty"`
+	Junk    bool   `json:"junk,omitempty"`
+}
+
+// IngestMailResp — what core did with the message. TaskIDs are the rows
+// CaptureTask returned; Created counts the ones that were new (a re-read
+// mailbox dedupes to Created=0). Skipped is set when nothing was asked of the
+// model at all — junk, or an empty message.
+//
+// Nothing here echoes the mail back. The reader logs counts.
+type IngestMailResp struct {
+	TaskIDs []int64 `json:"task_ids,omitempty"`
+	Created int     `json:"created"`
+	Skipped bool    `json:"skipped,omitempty"`
+}
+
 type listTasksReq struct {
 	Status string `json:"status"` // "" all | "live" | candidate|open|done|dropped
 }