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.

template/dashboard.html

@@ -138,7 +138,7 @@
                         <td>
                             {{if .ImageURL}}<img src="{{.ImageURL}}" class="ep-thumb" />{{else}}—{{end}}
                         </td>
-                        <td>{{.PublishedAt.Format "Jan 2, 2006"}}</td>
+                        <td>{{.PublishedAt}}</td>
                         <td>{{if .AudioPath}}✓{{else}}—{{end}}</td>
                         <td>
                             <div class="episode-actions">