Live-updating upload board, self-clearing tiles, original MD5 batch size

- MD5 auto-match now sends 75 md5: metatags per posts.json query, the same
  batch size the original J621-Django app used (was 20); the limit cap no
  longer truncates batches.
- every settled upload is upserted into the staged list right away, so the
  Pending / Visual Similarity / Auto-uploaded columns move as files land
  instead of waiting for the whole batch (auto-matched resolves and IQDB
  results use the same path).
- finished upload tiles fade out and remove themselves ~2s after completing;
  failures stay until cleared. Batch counters are tracked separately from the
  visible tiles so the header and progress bar stay accurate as tiles vanish.
- AGENTS.md now points at the original Django app for reference behavior.

Verified live with a headless upload run: the columns showed the new files
immediately and the 8 tiles were gone ~3s after finishing.

AGENTS.md

@@ -2,6 +2,11 @@ Use the venv on backend/venv/
 
 DO NOT mess with the system's python
 
+The original J621 (Django) app is kept for reference at
+    /mnt/Disco/Proyects/Python3.13/J621-Django/
+Consult it when matching original behavior (batch sizes for e621 lookups,
+per-page sets, upload flow) instead of guessing.
+
 For Frontend work:
 
 read both Frontend Design related Markdown files on the frontend/ folder