fix(importer): catch PIL OSError during transparency + phash blocks, skip as invalid_image instead of letting Celery autoretry loop forever

Operator hit a corrupt JPEG in the IR set 2026-05-25: PIL.verify() only
validates header structure but doesn't catch truncated/broken pixel
data. The error surfaces later in _transparency_pct (via getchannel
'A' -> load) or compute_phash (load) — both blow up with OSError
'broken data stream when reading image file'. Celery's autoretry_for
then bounces the same file forever instead of marking it skipped.

Wrap both PIL.load-triggering call sites with try/except OSError ->
ImportResult(status=skipped, skip_reason=invalid_image).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/services/importer.py

@@ -280,7 +280,18 @@ class Importer:
                 )
 
             if self.settings.skip_transparent and has_alpha:
-                pct = self._transparency_pct(source)
+                try:
+                    pct = self._transparency_pct(source)
+                except OSError as exc:
+                    # PIL.verify() at line 263 only validates header structure;
+                    # truncated/corrupt pixel data only surfaces when load()
+                    # actually decodes (here via getchannel('A')). Convert to
+                    # invalid_image skip so the Celery autoretry loop doesn't
+                    # bounce the same broken file forever.
+                    return ImportResult(
+                        status="skipped", skip_reason=SkipReason.invalid_image,
+                        error=f"PIL load failed during transparency check: {exc}",
+                    )
                 if pct >= self.settings.transparency_threshold:
                     return ImportResult(
                         status="skipped", skip_reason=SkipReason.too_transparent,
@@ -302,8 +313,16 @@ class Importer:
         # Perceptual near-dup (images only; videos keep phash NULL).
         phash = None
         if not is_video(source):
-            with Image.open(source) as im:
-                phash = compute_phash(im)
+            try:
+                with Image.open(source) as im:
+                    phash = compute_phash(im)
+            except OSError as exc:
+                # Same rationale as the transparency-check guard above:
+                # broken-pixel-data files pass verify() but blow up here.
+                return ImportResult(
+                    status="skipped", skip_reason=SkipReason.invalid_image,
+                    error=f"PIL load failed during phash compute: {exc}",
+                )
             if phash is not None:
                 cand_rows = self.session.execute(
                     select(