Stop storing completed uploads; announce them through a live feed

Every auto-matched, duplicate or manually resolved upload left a completed
TempUpload row on the board until it was dismissed by hand, so the rows
accumulated without bound and the bulk dismiss (capped at 1000 ids) failed
once there were more. The original app never stored these: they are
notifications, not records.

- complete_temp_upload now appends {filename, J-ID, resolution, post} to a
  bounded recent_completions feed on UploadRun and deletes the staged row
- staging duplicates never create a board record either; the create response
  carries the J-ID and preview so the SPA can show the card immediately
- status_payload returns the feed (newest first, signed thumbnails) for the
  live board; finalize_round counts deleted matches in processed
- resolve/link-bulk return synthetic completion payloads
- migration 0012 adds the field and purges the existing completed backlog
  (and any stray staged files) on deploy

backend/apps/library/models.py

@@ -229,6 +229,11 @@ class UploadRun(models.Model):
     matched = models.IntegerField(default=0)
     failed = models.IntegerField(default=0)
     error = models.TextField(blank=True, default="")
+    # Rolling feed of recent completions for the upload board. Completed
+    # staged uploads are deleted as soon as they are indexed; this only tells
+    # the live page "filename -> J-x" while it watches. Bounded, never
+    # dismissed, and ignored by fresh page loads.
+    recent_completions = models.JSONField(default=list, blank=True)
     started_at = models.DateTimeField(null=True, blank=True)
     updated_at = models.DateTimeField(auto_now=True)