feat(panel): move a delivery's details onto its own page

The delivery log now lists what identifies a message and nothing else —
time, sender, recipient, subject, status — and links each row to
/deliveries/{id}, which carries the rest: the sending domain, the
application it was submitted under, the Postfix queue id to search the
system log for, and when the status was last reported. Domain and
application were a column each; they were the widest thing in the table
after the addresses and repeat down every filtered page, and they remain
the log's two filters. Back returns to the page and filters the row was
opened from, rebuilt from the log's own parameters only.

Subjects are now decoded for display as well as on the way in. The milter
has decoded them since 8add005, but the rows it wrote before that still
hold the raw =?utf-8?Q?...?= header, and those are the ones an operator is
most likely to still be reading. The decoder moves to internal/mailhdr,
shared by the milter and the panel; it is idempotent, so a row decoded
once passes through unchanged.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

internal/store/sendlog.go

@@ -1,6 +1,8 @@
 package store
 
 import (
+	"database/sql"
+	"errors"
 	"fmt"
 	"strings"
 	"time"
@@ -104,6 +106,37 @@ type SendLogRow struct {
 	Subject   string
 	Status    string
 	CreatedAt time.Time
+	// UpdatedAt is when the status last changed — the log-tailer's report of
+	// the delivery attempt. It equals CreatedAt while a row is still queued.
+	UpdatedAt time.Time
+}
+
+// ErrSendLogNotFound is returned by GetSendLog for an id that no longer exists.
+// Send-log rows are pruned on the retention window, so a bookmarked delivery
+// disappearing is expected, not a fault.
+var ErrSendLogNotFound = errors.New("send-log entry not found")
+
+// GetSendLog returns a single send-log row by id, for the delivery detail page
+// the log links each row to.
+func (s *Store) GetSendLog(id int64) (SendLogRow, error) {
+	var (
+		row                  SendLogRow
+		createdAt, updatedAt string
+	)
+	err := s.db.QueryRow(
+		`SELECT id, queue_id, domain, app_login, from_addr, to_addr, subject, status, created_at, updated_at
+		 FROM send_log WHERE id = ?`, id,
+	).Scan(&row.ID, &row.QueueID, &row.Domain, &row.AppLogin, &row.From, &row.To,
+		&row.Subject, &row.Status, &createdAt, &updatedAt)
+	if errors.Is(err, sql.ErrNoRows) {
+		return SendLogRow{}, ErrSendLogNotFound
+	}
+	if err != nil {
+		return SendLogRow{}, fmt.Errorf("get send_log %d: %w", id, err)
+	}
+	row.CreatedAt, _ = time.Parse(time.RFC3339, createdAt)
+	row.UpdatedAt, _ = time.Parse(time.RFC3339, updatedAt)
+	return row, nil
 }
 
 // SendLogFilter narrows QuerySendLog/CountSendLog by domain and/or
@@ -119,7 +152,7 @@ func (s *Store) QuerySendLog(filter SendLogFilter, limit, offset int) ([]SendLog
 	where, args := sendLogWhere(filter)
 	args = append(args, limit, offset)
 	rows, err := s.db.Query(
-		`SELECT id, queue_id, domain, app_login, from_addr, to_addr, subject, status, created_at
+		`SELECT id, queue_id, domain, app_login, from_addr, to_addr, subject, status, created_at, updated_at
 		 FROM send_log`+where+`
 		 ORDER BY id DESC
 		 LIMIT ? OFFSET ?`,
@@ -133,14 +166,15 @@ func (s *Store) QuerySendLog(filter SendLogFilter, limit, offset int) ([]SendLog
 	var out []SendLogRow
 	for rows.Next() {
 		var (
-			row       SendLogRow
-			createdAt string
+			row                  SendLogRow
+			createdAt, updatedAt string
 		)
 		if err := rows.Scan(&row.ID, &row.QueueID, &row.Domain, &row.AppLogin,
-			&row.From, &row.To, &row.Subject, &row.Status, &createdAt); err != nil {
+			&row.From, &row.To, &row.Subject, &row.Status, &createdAt, &updatedAt); err != nil {
 			return nil, fmt.Errorf("scan send_log row: %w", err)
 		}
 		row.CreatedAt, _ = time.Parse(time.RFC3339, createdAt)
+		row.UpdatedAt, _ = time.Parse(time.RFC3339, updatedAt)
 		out = append(out, row)
 	}
 	return out, rows.Err()