perf(images): run image work on two own pools and trim after jobs (#523) (#582)

* perf(images): read PNG text without decoding pixels (#523)

Pillow's PngImageFile.text loads every pixel to reach text chunks written
after IDAT. The scan's metadata batch and the image monitor now read PNG
text through utils/png_text.py, which runs Pillow's own chunk handlers
after the first IDAT and seeks over the pixel data.

The legacy .text read is the floor: a differential test compares both
readers on every fixture image, a curated corpus and a seeded fuzz. The
only difference is a file whose pixel data is undecodable or cut short:
it now keeps the text read before its pixels instead of reading nothing.
Animated PNGs still use .text.

* test(images): cover the text budget, XMP and layout guard of the PNG text walk (#523)

* perf(images): run image work on two own pools and trim after jobs (#523)

Image work no longer spreads over the default executor's threads, where
glibc kept an image-sized malloc arena per thread.

- Interactive image work (view previews behind their gate, workspace card
  facts, both inspector routes) runs on the API's fixed pm-image pool of
  3 threads; the on-demand preview pool is merged into it.
- map_parallel (scan metadata, thumbnail and convert jobs, the legacy
  gallery generation, capture previews) shares one long-lived pm-worker
  pool of Worker threads, reused between jobs. The package teardown stops
  it within its bound (part "workers").
- After a scan, thumbnail or convert job, glibc's malloc_trim(0) returns
  free pages once; a no-op on Windows, macOS and musl.
- tools/perf/memory_drive.py measures RSS, heap, threads and arenas under
  the #523 load on the disposable browser test server (manual only).

* fix(images): start no image thread after the teardown; join retired job pools (#523)

py/api/admin.py

@@ -1454,6 +1454,7 @@ class AdminRoutesMixin:
             verb = "cancelled" if state == "cancelled" else "finished"
             job.log_line(f"Scan {verb} · {_plural(job.processed, 'file')} processed")
         job.finish(state)
+        await self._release_job_memory()
         if state == "finished":
             # After the scan, so the guarded thumbnail job is admitted (#249).
             await self._hand_off_scan_thumbnails(job, output_dirs)
@@ -1668,7 +1669,8 @@ class AdminRoutesMixin:
     def _extract_batch_metadata_sync(self, file_batch):
         """Extract metadata from a batch of files in one executor call.
 
-        Files are read on ``PromptManagerConfig.WORKER_THREADS`` threads.
+        Files are read on the long-lived job pool of
+        ``PromptManagerConfig.WORKER_THREADS`` threads (#523).
         PNG text is read chunk by chunk without decoding pixels (#523); file
         reads and ffprobe release the GIL.
         """