fix(images): fence the capture-date backfill on the file it read (#1201) (#1204)

The capture-date backfill committed its result keyed on the row id alone. It
snapshots every candidate up front, then walks them one at a time reading
originals off S3 or a NAS mount — a pass that can run for many minutes.

replacePhoto, reachable from the replace_by_name upload path, swaps a NEW file
under an existing row and rewrites path/filename. A replacement landing inside
that window carries no date of its own, so captured_at was still NULL, the
whereNull guard passed, and the previous file's EXIF date was written onto the
new photo. Silent: nothing errored, the run reported it as a success, and the
gallery just sorted that photo to the wrong place.

Fenced on path and filename as well as the id — the same fence #1199 put on the
orientation backfill for the same reason — so a replaced row matches zero rows
and is skipped. The candidate query already selects both columns, so no query
change. Knex renders a null value in the object form as `is null` on both the pg
and sqlite3 clients, so a row with a NULL path still matches itself.

Those skipped candidates are now counted rather than dropped. replacePhoto is
not the only writer of path/filename — eventRenameService rewrites both on an
event rename, which is not a content change — and another writer filling
captured_at first lands in the same place. Without a counter they fell out of
the run's arithmetic entirely: success + noExif + failed no longer added up to
the count the operator was shown when they started the job, on the card as well
as in the log.

The card shows the count only when it is non-zero, the same shape the
orientation job uses for staleTiers. The wording states what is known — changed
by something else, not updated — rather than promising a retry: for the
already-dated case there is nothing to retry, and the Missing Capture Date
figure above is what says whether work is left. Locale coverage matches the
staleTiers key (en, de, fr, sl), with the defaultValue carrying the rest.

Regression test: a replacement landing mid-run leaves captured_at NULL and is
not counted as updated. Verified to fail against the unfenced code.

backend/__tests__/integration/captureDateBackfill.test.js

@@ -198,5 +198,30 @@ describe('capture date backfill (#1172)', () => {
 
     expect(new Date((await db('photos').where({ id: photoId }).first()).captured_at).toISOString()).toBe(claimed);
     expect(done.body.lastResult.success).toBe(0);
+    // Read but not written, so it is accounted for rather than dropped.
+    expect(done.body.lastResult.skipped).toBe(1);
+  });
+
+  it('does not date a row whose file was replaced while it was reading (#1201)', async () => {
+    // replacePhoto swaps a NEW file under an existing row and rewrites
+    // path/filename (reachable from replace_by_name). The replacement carries
+    // no date of its own, so captured_at is still NULL and the whereNull guard
+    // alone would let the previous file's EXIF date land on it. The write is
+    // fenced on the identity that was read, so the row is skipped instead —
+    // and not counted as updated either.
+    const { photoId } = await seed({ relpath: 'orig.jpg', exifIso: '2026-06-03T09:00:00Z' });
+
+    const res = await request(app).post('/api/admin/photos/repair-capture-dates');
+    expect(res.body.count).toBe(1);
+    // Simulate the replacement landing before the loop writes.
+    await db('photos').where({ id: photoId })
+      .update({ path: 'capfill/replaced.jpg', filename: 'replaced.jpg' });
+    const done = await settle();
+
+    expect((await db('photos').where({ id: photoId }).first()).captured_at).toBeNull();
+    expect(done.body.lastResult.success).toBe(0);
+    // Not an error and not "no EXIF" — the date was found, another writer just
+    // got there first. It stays in the backlog for the next run.
+    expect(done.body.lastResult).toMatchObject({ noExif: 0, failed: 0, skipped: 1 });
   });
 });