start_monitoring assigned self.observer before checking that any directory
existed, so with no valid directory the monitor was stuck "already running"
and stop_monitoring raised on join of a never-started observer. Directories
are validated first and only a started observer is kept; stop_monitoring is
safe to call at any time and always joins. New is_monitoring property.
The per-image threading.Timer is replaced by one daemon worker: paths are
deduplicated and bounded (MAX_PENDING_IMAGES), processed in creation order
(batch images pop the prompt queue in order) after the processing delay and
once the file size is unchanged across two reads.
Vito Sansevero
59ad23201b