Capture tasks, with one intake seam mail can call later (#130)

A task is not a fact and not a note. A fact is a claim about the world that a
correction supersedes; a note is something to recall by meaning. A task is work
with a lifecycle, and the read that matters is "everything outstanding right
now" — which over an append-only log would mean replaying history on every
question. So: a tasks table, migration #14, statuses candidate/open/done/dropped
that each move forward exactly once.

Dedupe is on normalised text among LIVE rows only, via a partial unique index.
That is the property the mail side needs: an extractor may call CaptureTask for
every message it reads, as often as it likes, without growing the list — while a
weekly errand is still capturable again once the last one is done.

Three ways in, one seam. ipc.CaptureTaskReq is it: the voice path
(router.ParseTaskCapture on an explicit marker — "добавь в задачи …", never
"надо бы поспать"), the /tasks form, and the email reader from #246 when it
exists. Mail-derived items set Source "email:<account>", Status "candidate" and
Evidence to whatever makes the row reviewable; a candidate is inert until he
confirms it on /tasks, and Maven names it as unconfirmed when she recites the
list rather than putting words in his mouth.

No new intent — the router enum is a contract with the relabelling prompt, so
capture rides the note intent and the list rides a query source, both matched
deterministically like the calendar and plan matchers already are.

Nothing here speaks. No tick rule reads tasks; the list is answered when asked
about, which is why /tasks POST is not step-up gated the way /tools and
/routines are — a task write moves no boundary.

Vikunja #130

internal/ipc/unimplemented.go

@@ -89,6 +89,15 @@ func (UnimplementedCoreAPI) DisableTool(ctx context.Context, name string) error
 func (UnimplementedCoreAPI) DeleteTool(ctx context.Context, name string) error {
 	return ErrNotImplemented
 }
+func (UnimplementedCoreAPI) CaptureTask(ctx context.Context, req CaptureTaskReq) (CaptureTaskResp, error) {
+	return CaptureTaskResp{}, ErrNotImplemented
+}
+func (UnimplementedCoreAPI) ListTasks(ctx context.Context, status string) ([]Task, error) {
+	return nil, ErrNotImplemented
+}
+func (UnimplementedCoreAPI) SetTaskStatus(ctx context.Context, id int64, status string, ts time.Time) error {
+	return ErrNotImplemented
+}
 func (UnimplementedCoreAPI) ListProposedRoutines(ctx context.Context) ([]ProposedRoutine, error) {
 	return nil, ErrNotImplemented
 }