diff --git a/backend/app/tasks/maintenance.py b/backend/app/tasks/maintenance.py
index 05dcc4e..1572406 100644
--- a/backend/app/tasks/maintenance.py
+++ b/backend/app/tasks/maintenance.py
@@ -1,16 +1,22 @@
 """Periodic maintenance: recover stuck import tasks, garbage-collect old finished tasks."""
 
+import logging
 from datetime import UTC, datetime, timedelta
 
+from PIL import Image
 from sqlalchemy import create_engine, delete, select, update
 from sqlalchemy.orm import sessionmaker
 
 from ..celery_app import celery
 from ..config import get_config
-from ..models import ImportTask
+from ..models import ImageRecord, ImportTask
+from ..utils.phash import compute_phash
+
+log = logging.getLogger(__name__)
 
 STUCK_THRESHOLD_MINUTES = 30
 OLD_TASK_DAYS = 7
+PHASH_PAGE = 500
 
 
 def _sync_session_factory():
@@ -71,3 +77,41 @@ def cleanup_old_tasks() -> int:
         )
         session.commit()
         return result.rowcount or 0
+
+
+@celery.task(name="backend.app.tasks.maintenance.backfill_phash")
+def backfill_phash() -> int:
+    """Recompute phash for stored images that have none (imported before
+    FC-2d-i+ii). Keyset-paginated by id (restart-safe), NULL-only fill,
+    idempotent. Videos legitimately keep phash NULL. A missing/unreadable
+    file is logged and left NULL — never fails the task."""
+    SessionLocal = _sync_session_factory()
+    updated = 0
+    last_id = 0
+    with SessionLocal() as session:
+        while True:
+            rows = session.execute(
+                select(ImageRecord)
+                .where(ImageRecord.id > last_id)
+                .where(ImageRecord.phash.is_(None))
+                .where(ImageRecord.mime.like("image/%"))
+                .order_by(ImageRecord.id.asc())
+                .limit(PHASH_PAGE)
+            ).scalars().all()
+            if not rows:
+                break
+            for rec in rows:
+                try:
+                    with Image.open(rec.path) as im:
+                        ph = compute_phash(im)
+                except Exception as exc:
+                    log.warning(
+                        "backfill_phash: unreadable %s: %s", rec.path, exc
+                    )
+                    ph = None
+                if ph is not None and rec.phash is None:
+                    rec.phash = ph
+                    updated += 1
+            session.commit()
+            last_id = rows[-1].id
+    return updated
diff --git a/tests/test_backfill_phash.py b/tests/test_backfill_phash.py
new file mode 100644
index 0000000..b16d1ac
--- /dev/null
+++ b/tests/test_backfill_phash.py
@@ -0,0 +1,81 @@
+"""FC-2d-vi: library-wide phash backfill (keyset, idempotent)."""
+
+import pytest
+from PIL import Image
+
+from backend.app.models import ImageRecord
+from backend.app.tasks.maintenance import backfill_phash
+
+pytestmark = pytest.mark.integration
+
+
+class _Ctx:
+    def __init__(self, s):
+        self.s = s
+
+    def __enter__(self):
+        return self.s
+
+    def __exit__(self, *a):
+        return False
+
+
+def _sf(db_sync):
+    """sessionmaker-like returning the test's bound session, so the task
+    runs against the same transaction / sees the seeded rows."""
+    class _SM:
+        def __call__(self):
+            return _Ctx(db_sync)
+
+    return _SM()
+
+
+def _img(path, color):
+    path.parent.mkdir(parents=True, exist_ok=True)
+    Image.new("RGB", (64, 64), color).save(path, "JPEG")
+
+
+def _rec(db_sync, path, *, sha, mime="image/jpeg", phash=None):
+    rec = ImageRecord(
+        path=str(path), sha256=sha, phash=phash, size_bytes=1,
+        mime=mime, width=64, height=64, origin="imported_filesystem",
+        integrity_status="unknown",
+    )
+    db_sync.add(rec)
+    db_sync.flush()
+    return rec
+
+
+def test_backfill_fills_null_image_phash(db_sync, tmp_path, monkeypatch):
+    from backend.app.tasks import maintenance as m
+
+    p = tmp_path / "a.jpg"
+    _img(p, (10, 120, 200))
+    pre = tmp_path / "preset.jpg"
+    _img(pre, (5, 5, 5))
+
+    rec = _rec(db_sync, p, sha="a" + "0" * 63, phash=None)
+    vid = _rec(
+        db_sync, tmp_path / "v.mp4", sha="v" + "0" * 63,
+        mime="video/mp4", phash=None,
+    )
+    missing = _rec(
+        db_sync, tmp_path / "gone.jpg", sha="g" + "0" * 63, phash=None,
+    )
+    preset = _rec(
+        db_sync, pre, sha="p" + "0" * 63, phash="deadbeefdeadbeef",
+    )
+    db_sync.commit()
+
+    monkeypatch.setattr(m, "_sync_session_factory", lambda: _sf(db_sync))
+
+    updated = backfill_phash()
+    db_sync.expire_all()
+    assert updated == 1
+    assert db_sync.get(ImageRecord, rec.id).phash is not None
+    assert db_sync.get(ImageRecord, vid.id).phash is None
+    assert db_sync.get(ImageRecord, missing.id).phash is None
+    assert db_sync.get(ImageRecord, preset.id).phash == "deadbeefdeadbeef"
+
+    # idempotent: a second run touches nothing
+    assert backfill_phash() == 0