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/import.go

@@ -243,7 +243,7 @@ func (h *ImportHandler) importSelected(w http.ResponseWriter, r *http.Request, r
 			publishedAt = t
 		}
 
-		_, err = h.Store.CreateEpisode(userID, item.Title, item.Description, audioPath, imagePath, publishedAt)
+		_, err = h.Store.CreateEpisode(userID, item.Title, item.Description, audioPath, imagePath, publishedAt.Format("2006-01-02 15:04:05"))
 		if err != nil {
 			errors = append(errors, fmt.Sprintf("%s: database error: %v", item.Title, err))
 			h.removeFile(audioPath)