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.

main.go

@@ -14,6 +14,7 @@ import (
 	"time"
 
 	"pete/internal/config"
+	"pete/internal/feedwatch"
 	"pete/internal/ingestion"
 	"pete/internal/matrix"
 	"pete/internal/poster"
@@ -243,6 +244,15 @@ func main() {
 	// Start pollers
 	poller := ingestion.NewPoller(cfg.Sources, processItem)
 	poller.Start(ctx)
+
+	// Tell the owner when a feed stops fetching, comes back, or goes quiet.
+	// Without an admin room there is nowhere to say it, and marking alerts as
+	// sent into the void would mean they never arrive once a room is set.
+	if mx.HasAdminRoom() {
+		go feedwatch.Run(ctx, cfg.Sources, mx.SendAdminNotice)
+	} else {
+		slog.Warn("feed alerts off: set matrix.admin_room to have Pete report broken or quiet feeds")
+	}
 	slog.Info("pollers started")
 
 	if postingEnabled && roundRobinMode {