feedwatch: tell the owner when a feed breaks, comes back, or goes quiet

Every poll's outcome has always been written to source_health and shown on
/status, but only to someone who went and looked. Nobody did: Siliconera has
been refused on 3,769 polls in a row and Digital Foundry on 1,900, and
neither has ever fetched successfully. SendAdminWarning existed and nothing
called it.

A watcher now reads source_health every 15 minutes and sends one notice to
matrix.admin_room covering everything that changed since it last spoke:
feeds that have failed 5 polls in a row, feeds that have fetched since they
were reported down, and feeds that fetch fine but have kept nothing new for 7
days (every healthy source produces something inside about 4). What has been
said is stored on the row (down_alerted_at, quiet_alerted_at), so each
episode is reported once and a restart doesn't repeat it. A notice that
fails to send is not marked, and goes out on the next pass.

last_new_at is new, moved only when a poll keeps a story. Existing rows start
from their newest retained story, or from now if they have none, and a new
source starts at its first poll: nothing on disk says whether a source with
no stories is long dead or was added yesterday, so it gets a full window
before it can be called quiet.

With no admin room configured the watcher doesn't run at all, rather than
marking alerts as delivered to nowhere.

internal/matrix/client.go

@@ -508,23 +508,26 @@ func (c *Client) sendImage(ctx context.Context, roomID id.RoomID, imageURL strin
 	return err
 }
 
-// SendAdminWarning sends a warning message to the admin room, if configured.
-func (c *Client) SendAdminWarning(msg string) {
+// SendAdminNotice sends a notice to the admin room. It returns an error when no
+// admin room is configured or the send fails, so a caller that must not lose
+// the message (the feed watcher) can try again later.
+func (c *Client) SendAdminNotice(ctx context.Context, msg string) error {
 	if c.adminRoom == nil {
-		slog.Warn("admin warning (no admin room configured)", "msg", msg)
-		return
+		return fmt.Errorf("no admin room configured")
 	}
-
 	content := &event.MessageEventContent{
-		MsgType: event.MsgText,
+		MsgType: event.MsgNotice,
 		Body:    msg,
 	}
-
-	if _, err := c.mx.SendMessageEvent(context.Background(), *c.adminRoom, event.EventMessage, content); err != nil {
-		slog.Error("failed to send admin warning", "err", err, "msg", msg)
+	if _, err := c.mx.SendMessageEvent(ctx, *c.adminRoom, event.EventMessage, content); err != nil {
+		return fmt.Errorf("send admin notice: %w", err)
 	}
+	return nil
 }
 
+// HasAdminRoom reports whether an admin room is configured.
+func (c *Client) HasAdminRoom() bool { return c.adminRoom != nil }
+
 // ChannelRoomID returns the room ID for a named channel.
 // Returns false if the channel is not configured or has an empty room ID.
 func (c *Client) ChannelRoomID(channel string) (id.RoomID, bool) {