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/db/migrations/0074_notification_email.up.sql

@@ -0,0 +1,54 @@
+-- M489 #5346: notifications by email, grouped. Nothing is emailed per event.
+--
+-- New music (request_completed) goes out as at most one summary a day, at a set
+-- hour in each user's own timezone. Everything else is batched: a batch opens
+-- at the first un-emailed item and one email goes out a window later, holding
+-- whatever accumulated. internal/notifications/digest.go is the one sender.
+
+-- The two knobs, in admin Settings (rule 25). Singleton in the style of
+-- fingerprint_settings (0061).
+CREATE TABLE notification_email_settings (
+    id                    boolean PRIMARY KEY DEFAULT true,
+    -- The local hour (0-23, in each user's timezone) the daily new-music
+    -- summary goes out.
+    summary_hour          integer NOT NULL DEFAULT 9,
+    -- How long a batch stays open after its first item before it is sent.
+    batch_window_minutes  integer NOT NULL DEFAULT 60,
+    updated_at            timestamptz NOT NULL DEFAULT now(),
+
+    CONSTRAINT notification_email_settings_singleton CHECK (id = true),
+    CONSTRAINT notification_email_settings_hour_range
+        CHECK (summary_hour >= 0 AND summary_hour <= 23),
+    CONSTRAINT notification_email_settings_window_range
+        CHECK (batch_window_minutes >= 15 AND batch_window_minutes <= 1440)
+);
+INSERT INTO notification_email_settings (id) VALUES (true) ON CONFLICT (id) DO NOTHING;
+
+-- Per user, per email group: where the digest stands. A missing row is a user
+-- who has never had a batch open or a summary sent.
+CREATE TABLE user_notification_email_state (
+    user_id          uuid        NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    email_group      text        NOT NULL CHECK (email_group IN ('batch', 'summary')),
+    -- When the open batch started. Held here rather than read off the items,
+    -- because a coalesced item moves its created_at forward on every update
+    -- and would otherwise keep a batch from ever coming due.
+    batch_opened_at  timestamptz,
+    -- The last email of this group the mailer accepted. A summary is due once
+    -- per local day, after the summary hour, if this is before it.
+    last_sent_at     timestamptz,
+    -- A failed send backs off: nothing is tried for this group before
+    -- retry_after, and the gap doubles with each failure in a row.
+    failures         integer     NOT NULL DEFAULT 0,
+    retry_after      timestamptz,
+    PRIMARY KEY (user_id, email_group)
+);
+
+-- The digest's selection: unread rows not yet emailed.
+CREATE INDEX user_notifications_email_pending_idx
+    ON user_notifications (user_id, created_at)
+    WHERE read_at IS NULL AND emailed_at IS NULL;
+
+-- Rows written before this migration were never judged for email. Treat them
+-- as already handled, so the first digest after the upgrade doesn't send a
+-- backlog nobody asked for.
+UPDATE user_notifications SET emailed_at = now() WHERE emailed_at IS NULL;