feat(notifications): grouped email digest, new music as a daily summary (#5346)

Nothing is emailed per event. New music (request_completed) goes out at
most once a day, at the summary hour in each user's own timezone, grouped
by artist. Everything else is batched: one email a window after the first
un-emailed item, holding whatever accumulated.

- Migration 0074: notification_email_settings (summary hour, batch window,
  admin-configurable) and user_notification_email_state (batch start, last
  sent, failures and retry_after per user and group). Existing rows are
  stamped emailed so the upgrade sends no backlog.
- The Notifier stamps emailed_at at write time when the recipient's email
  channel is off, so turning email on later doesn't send old items.
- Read rows are never selected. A row is stamped only after the mailer
  accepts, in one transaction with the state, against the read's clock, so
  a coalesced row updated mid-send stays pending.
- A failed send backs off 5m doubling to 6h; SMTP not configured just waits.
- Links come from the public address; without one the email has none.
- The mailer now RFC 2047-encodes subjects and strips line breaks from them.
- Admin → Integrations gains a Notification emails card.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/notifications/digest_render.go

@@ -0,0 +1,120 @@
+package notifications
+
+import (
+	"bytes"
+	"embed"
+	"encoding/json"
+	"fmt"
+	htmltemplate "html/template"
+	"strings"
+	texttemplate "text/template"
+)
+
+//go:embed templates/digest.txt templates/digest.html
+var digestFS embed.FS
+
+var (
+	digestText = texttemplate.Must(texttemplate.ParseFS(digestFS, "templates/digest.txt"))
+	digestHTML = htmltemplate.Must(htmltemplate.ParseFS(digestFS, "templates/digest.html"))
+)
+
+// digestEmail is one rendered email.
+type digestEmail struct {
+	Subject string
+	Text    string
+	HTML    string
+}
+
+type digestLine struct {
+	Title string
+	Body  string
+	URL   string
+}
+
+type digestArtist struct {
+	Name   string
+	URL    string
+	Titles []digestLine
+}
+
+type digestVars struct {
+	Name        string
+	Intro       string
+	Items       []digestLine
+	Artists     []digestArtist
+	SettingsURL string
+}
+
+// renderDigest renders one group's email. base is the public address; with
+// none set, the email still goes out, without links.
+func renderDigest(group EmailGroup, name string, items []pendingItem, base string) (digestEmail, error) {
+	vars := digestVars{Name: name, SettingsURL: joinURL(base, "/settings#notifications")}
+	var subject string
+	if group == EmailSummary {
+		vars.Artists = summaryArtists(items, base)
+		vars.Intro = "New in your library since the last summary:"
+		subject = "Minstrel: new music in your library"
+	} else {
+		for _, it := range items {
+			r := Render(it.Kind, it.Payload)
+			vars.Items = append(vars.Items, digestLine{Title: r.Title, Body: r.Body, URL: joinURL(base, r.Link)})
+		}
+		vars.Intro = "Here's what happened on Minstrel:"
+		subject = fmt.Sprintf("Minstrel: %d updates", len(items))
+		if len(items) == 1 {
+			subject = "Minstrel: " + vars.Items[0].Title
+		}
+	}
+
+	var text, html bytes.Buffer
+	if err := digestText.Execute(&text, vars); err != nil {
+		return digestEmail{}, fmt.Errorf("render text: %w", err)
+	}
+	if err := digestHTML.Execute(&html, vars); err != nil {
+		return digestEmail{}, fmt.Errorf("render html: %w", err)
+	}
+	return digestEmail{Subject: subject, Text: text.String(), HTML: html.String()}, nil
+}
+
+// summaryArtists groups new-music arrivals by artist, in the order each
+// artist first arrived, with each artist's albums and tracks under them. An
+// artist request has no title of its own; the artist links to its page.
+func summaryArtists(items []pendingItem, base string) []digestArtist {
+	var out []digestArtist
+	index := map[string]int{}
+	for _, it := range items {
+		var p Payload
+		_ = json.Unmarshal(it.Payload, &p)
+		artist, title := p.Artist, p.Title
+		if artist == "" {
+			// Rows from before Artist was recorded carry only "Artist – Title".
+			artist, title, _ = strings.Cut(p.Name, " – ")
+		}
+		if artist == "" {
+			artist = "Something you asked for"
+		}
+		i, ok := index[artist]
+		if !ok {
+			i = len(out)
+			index[artist] = i
+			out = append(out, digestArtist{Name: artist})
+		}
+		link := Render(it.Kind, it.Payload).Link
+		if title == "" {
+			if p.ArtistID != "" {
+				out[i].URL = joinURL(base, link)
+			}
+			continue
+		}
+		out[i].Titles = append(out[i].Titles, digestLine{Title: title, URL: joinURL(base, link)})
+	}
+	return out
+}
+
+// joinURL is base followed by path, or "" when no public address is set.
+func joinURL(base, path string) string {
+	if base == "" || path == "" {
+		return ""
+	}
+	return strings.TrimRight(base, "/") + path
+}