Add Dockerfile, fix SQLite immutable mode, URL-encode credentials

- Dockerfile: multi-stage build with golang:1.26 and alpine + ffmpeg
- SQLite: use file: URI with immutable=1 for read-only access
- URL builder: encode user/pass with PathEscape/QueryEscape for
  special characters (@, \, :, etc.)
- Health endpoint: truncate uptime to seconds
- Release skill: update smoke test to /api endpoint
- Remove unused ValidateID function

internal/api/api.go

@@ -108,7 +108,7 @@ func apiHandler(w http.ResponseWriter, r *http.Request) {
 func apiHealth(w http.ResponseWriter, r *http.Request) {
 	ResponseJSON(w, map[string]any{
 		"version": app.Version,
-		"uptime":  time.Since(app.StartTime).String(),
+		"uptime":  time.Since(app.StartTime).Truncate(time.Second).String(),
 	})
 }