feat(importer): _supersede() now applies the new (larger) file's sidecar — operator wanted to scan GS download dir to supersede smaller IR-migrated images AND wire up gallery-dl Post metadata, but supersede was file-only and silently dropped the sidecar.

_apply_sidecar is additive: it find-or-creates Post/Source/ImageProvenance
and sets primary_post_id NULL-only, so any IR-migration provenance on the
existing row survives untouched and the new GS sidecar adds a second
ImageProvenance pointing at the freshly-created Post.

Wrapped in try/except so a malformed sidecar can't unwind the file-swap
commit — the file replacement is the critical operation.

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

tests/test_phash_dedup.py

@@ -10,7 +10,15 @@ import pytest
 from PIL import Image
 from sqlalchemy import func, select
 
-from backend.app.models import ImageRecord, ImportSettings, Tag, TagKind
+from backend.app.models import (
+    ImageProvenance,
+    ImageRecord,
+    ImportSettings,
+    Post,
+    Source,
+    Tag,
+    TagKind,
+)
 from backend.app.models.tag import image_tag
 from backend.app.services.importer import Importer, SkipReason
 from backend.app.services.thumbnailer import Thumbnailer
@@ -141,6 +149,67 @@ def test_smaller_existing_is_superseded(importer, import_layout):
     assert Path(row.path).exists()
 
 
+def test_supersede_applies_new_file_sidecar(importer, import_layout):
+    """Operator-flagged 2026-05-25: scanning the GS download dir should
+    supersede smaller IR-migrated images AND wire up the GS sidecar's
+    Post/Source/ImageProvenance. Previously _supersede swapped the file
+    but ignored the sidecar entirely."""
+    import json
+    import_root, _ = import_layout
+
+    # Stage 1: a small, sidecar-less image (the "IR migration" precondition).
+    small = import_root / "ir-migration-folder" / "small.png"
+    _write(small, (60, 130, 200), (200, 200))
+    r1 = importer.import_one(small)
+    assert r1.status == "imported"
+    eid = r1.image_id
+
+    # Stage 2: a larger version of the same image (same phash) WITH a
+    # gallery-dl JSON sidecar adjacent. Live in a separate folder to
+    # simulate the GS download dir.
+    big = import_root / "Maewix" / "patreon" / "01_big.png"
+    _write(big, (60, 130, 200), (900, 900))
+    sidecar_path = big.with_suffix(big.suffix + ".json")
+    sidecar_path.parent.mkdir(parents=True, exist_ok=True)
+    sidecar_path.write_text(json.dumps({
+        "category": "patreon",
+        "id": 555,
+        "url": "https://www.patreon.com/posts/555",
+        "title": "Set 1",
+        "content": "<p>The big version</p>",
+        "page_count": 1,
+        "published_at": "2025-08-01T00:00:00+00:00",
+        "artist": "Maewix",
+    }))
+
+    r2 = importer.import_one(big)
+    assert r2.status == "superseded"
+    assert r2.image_id == eid
+
+    importer.session.expire_all()
+    # Row preserved, file replaced, sidecar metadata wired up.
+    row = importer.session.get(ImageRecord, eid)
+    assert row.width == 900 and row.height == 900
+
+    post = importer.session.execute(
+        select(Post).where(Post.external_post_id == "555")
+    ).scalar_one()
+    assert post.post_title == "Set 1"
+    assert "big version" in (post.description or "")
+
+    source = importer.session.execute(
+        select(Source).where(Source.id == post.source_id)
+    ).scalar_one()
+    assert source.platform == "patreon"
+
+    prov_count = importer.session.execute(
+        select(func.count(ImageProvenance.id))
+        .where(ImageProvenance.image_record_id == eid)
+        .where(ImageProvenance.post_id == post.id)
+    ).scalar_one()
+    assert prov_count == 1
+
+
 def test_threshold_controls_match(importer, import_layout):
     # Structurally distinct images (orthogonal splits) are far apart in
     # phash space, so a tight threshold keeps them independent rather than