feat(store,runner,web): channel unavailability notice (migration 013)

YouTube channels that 404 on playlist discovery (deleted/private) are now:
1. Wrapped in domain.ErrChannelUnavailable by the YouTube adapter (instead of
   a generic error), so the runner can identify them without string-matching.
2. Stored per-user in channel_errors (migration 013, RLS-guarded) via runner's
   new UpsertChannelError path — removed from the generic Errors counter,
   counted separately as ChannelUnavailable.
3. Shown on the account page under "Unavailable channels" with name, chip-warn
   badge, and first-seen date, so users know why some subscribed channels
   produce no videos.

internal/web/account.go

@@ -33,7 +33,12 @@ func (a *App) handleAccount(w http.ResponseWriter, r *http.Request) {
 		a.serverError(w, r, "summarize mode", err)
 		return
 	}
-	a.render(w, r, AccountPage(name, email, conns, auto, takeFlash(w, r)))
+	channelErrs, err := a.Store.ListChannelErrors(r.Context(), userID)
+	if err != nil {
+		a.serverError(w, r, "channel errors", err)
+		return
+	}
+	a.render(w, r, AccountPage(name, email, conns, auto, channelErrs, takeFlash(w, r)))
 }
 
 // handleDisconnect removes a provider connection: it deletes the OAuth token from