store, mavweb: the delivery outbox can be read (V-390)

The table was write-only. Rows were recorded and nothing could show them, so
the tests for #368 and #370 had to reach past the store into store.DB — if a
test can only see it that way, so can nobody else. A durable record nobody
reads answers no question, and why Maven went quiet is supposed to be a query.

ListDeliveryAttempts returns recent rows newest first, filtered by status.
Status is the filter worth having because the two real questions are "what got
dropped" and "what is still pending", and neither is answerable by reading the
whole list on a busy day. It reaches mavweb over IPC as DeliveryAttempts.

The section goes on /notifications, which already answers "what did she send",
rather than on a page of its own. Shared ui.css, the nav partial, the table in
div.scroll. A failed outbox read leaves a log line and still renders the nudge
list, because half the page beats none of it.

internal/ipc/api.go

@@ -60,6 +60,19 @@ type Nudge struct {
 	OutcomeTs *int64    `json:"outcome_ts,omitempty"`
 }
 
+// DeliveryAttempt — one row of the delivery outbox. Times are formatted by the
+// reader; Completed is nil while the attempt is still pending.
+type DeliveryAttempt struct {
+	ID         int64      `json:"id"`
+	Kind       string     `json:"kind"`
+	Rule       string     `json:"rule,omitempty"`
+	ReminderID int64      `json:"reminder_id,omitempty"`
+	Channel    string     `json:"channel"`
+	Status     string     `json:"status"`
+	Created    time.Time  `json:"created"`
+	Completed  *time.Time `json:"completed,omitempty"`
+}
+
 // Note — a recall/preference item; ranked by embedding cosine on query.
 // Score is set by QueryNotes (0 on the write path).
 type Note struct {
@@ -521,6 +534,12 @@ type outcomesReq struct {
 type nReq struct {
 	N int `json:"n"`
 }
+
+// deliveryAttemptsReq — the outbox read. Status is empty for every status.
+type deliveryAttemptsReq struct {
+	Status string `json:"status,omitempty"`
+	N      int    `json:"n"`
+}
 type kindNReq struct {
 	Kind string `json:"kind"`
 	N    int    `json:"n"`
@@ -685,6 +704,9 @@ type CoreAPI interface {
 	RecentActiveFactsByKind(ctx context.Context, kind string, n int) ([]Fact, error)
 	CalendarEvents(ctx context.Context, from, to time.Time) ([]Fact, error)
 	RecentNudges(ctx context.Context, n int) ([]Nudge, error)
+	// DeliveryAttempts reads the outbox, newest first. An empty status means
+	// every status (Vikunja #390).
+	DeliveryAttempts(ctx context.Context, status string, n int) ([]DeliveryAttempt, error)
 
 	// RecentEcosystemTraces reads the ecosystem call log, which lives in its
 	// own table so machine-rate traces never crowd out human-rate facts.