Slug-based RSS URLs and success flash messages

- Add podcast_slug to users with global uniqueness check
- RSS available at /rss/{slug} (path-based) with legacy ?slug= fallback
- Slug field in dashboard settings with validation (lowercase, hyphens)
- JS flash messages for success/error after form submissions
- Remove unused imports

main.go

@@ -75,8 +75,9 @@ func main() {
 	http.HandleFunc("/episodes/delete", authMid.RequireAuth(epH.ServeDelete))
 	http.HandleFunc("/settings", authMid.RequireAuth(setH.ServeSettings))
 
-	// Public RSS feed — per-user via ?user=N query param.
+	// Public RSS: /rss/my-slug or /rss?slug=my-slug or /rss?user=N (legacy)
 	http.HandleFunc("/rss", rssH.ServeRSS)
+	http.HandleFunc("GET /rss/{slug}", rssH.ServeRSS)
 
 	// Public uploaded files.
 	http.HandleFunc("/uploads/", epH.ServeUpload)