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/storage/db.go

@@ -96,6 +96,14 @@ func runMigrations(d *sql.DB) error {
 	addColumnIfMissing(d, "post_log", "url_canonical", "TEXT")
 	addColumnIfMissing(d, "post_log", "forced", "INTEGER NOT NULL DEFAULT 0")
 	addColumnIfMissing(d, "round_robin_state", "last_channel", "TEXT")
+	// Feed alerts. last_new_at is when a source last produced a story Pete kept;
+	// the two *_alerted_at stamps remember that the owner has already been told a
+	// feed is down or has gone quiet, so an alert fires once per episode and
+	// survives a restart. See backfillLastNewAt for what existing rows start with.
+	addColumnIfMissing(d, "source_health", "last_new_at", "INTEGER")
+	addColumnIfMissing(d, "source_health", "down_alerted_at", "INTEGER")
+	addColumnIfMissing(d, "source_health", "quiet_alerted_at", "INTEGER")
+	backfillLastNewAt(d)
 	// Occupancy of a shared table. Rows written before the casino went multiplayer
 	// are solo games and read as NULL, which is exactly what they are.
 	addColumnIfMissing(d, "game_live_hands", "table_id", "TEXT")