Fix dashboard failing to load episodes after RSS import

The root cause was that modernc.org/sqlite stores time.Time as strings
but cannot scan strings back into time.Time, causing ListEpisodes to
fail. Changed PublishedAt and CreatedAt in store structs from time.Time
to string, with format/parse handled at the application level.

Also fixed a secondary issue where the published_at format stored during
import had a duplicated timezone offset.

handler/rss.go

@@ -104,6 +104,10 @@ func (h *RSSHandler) ServeRSS(w http.ResponseWriter, r *http.Request) {
 			itemImage = &rssItunesImage{Href: h.fullURL(epImage)}
 		}
 
+		pubStr := ep.PublishedAt
+		if pt, err := time.Parse("2006-01-02 15:04:05", ep.PublishedAt); err == nil {
+			pubStr = pt.Format(time.RFC1123Z)
+		}
 		items = append(items, rssItem{
 			Title:       ep.Title,
 			Description: ep.Description,
@@ -111,7 +115,7 @@ func (h *RSSHandler) ServeRSS(w http.ResponseWriter, r *http.Request) {
 				URL:  h.fullURL(ep.AudioPath),
 				Type: "audio/mpeg",
 			},
-			PubDate:     ep.PublishedAt.Format(time.RFC1123Z),
+			PubDate:     pubStr,
 			GUID:        h.fullURL(ep.AudioPath),
 			ItunesImage: itemImage,
 		})