fix(dates): resolve IFD0 ModifyDate as a last-resort capture date

Scans, edited exports and some downloads carry only EXIF DateTime
(exiftool: IFD0:ModifyDate). The kamadak ingest path already takes it
as the capture date, but the exiftool resolver didn't ask for it, so
the date_utc drain resolved those rows to fs_time and then (rightly)
refused to overwrite their tag-sourced legacy date. 147 prod rows sat
on the legacy convention forever, re-running exiftool every tick.

ModifyDate is now the lowest-priority tag, and only from IFD0 — XMP /
QuickTime ModifyDate is an edit or mux time, not capture.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/date_resolver.rs

@@ -322,6 +322,13 @@ pub fn resolve_dates_batch(
 /// timestamps. `ContentCreateDate` is Apple's iOS-set tag; it often
 /// reflects local capture time on iPhone exports better than the others.
 ///
+/// `ModifyDate` comes last and only from `IFD0` (see
+/// [`MODIFY_DATE_GROUP`]): that's EXIF `DateTime`, which the kamadak
+/// ingest path already takes as the capture date when nothing better
+/// exists. Scans, edits and some downloads carry only that tag, and
+/// without it here the drain resolved them to fs_time and then refused
+/// to overwrite their tag-sourced legacy date — stuck forever.
+///
 /// Notably absent: `FileModifyDate` / `FileAccessDate` — those are
 /// filesystem-derived and the resolver covers them via the `fs_time`
 /// fallback. Letting exiftool pull them here would mask "no real EXIF
@@ -333,8 +340,14 @@ const EXIFTOOL_DATE_TAGS: &[&str] = &[
     "MediaCreateDate",
     "TrackCreateDate",
     "ContentCreateDate",
+    "ModifyDate",
 ];
 
+/// The only group whose `ModifyDate` counts as a date: `IFD0`, i.e. EXIF
+/// `DateTime`. XMP / QuickTime `ModifyDate` record when an editor or
+/// muxer last touched the file, which isn't a capture time.
+const MODIFY_DATE_GROUP: &str = "IFD0";
+
 /// Offset + GPS evidence tags requested in the same exiftool pass. They
 /// feed the camera-offset cascade for naive wall-clock readings that come
 /// from photo-EXIF groups (ADR-0001).
@@ -735,6 +748,9 @@ fn resolve_exiftool_payload(
     });
 
     for cand in candidates {
+        if cand.tag == "ModifyDate" && cand.group != MODIFY_DATE_GROUP {
+            continue;
+        }
         let Some(reading) = parse_exiftool_date_string(&cand.value) else {
             continue;
         };
@@ -1063,6 +1079,55 @@ mod tests {
         assert_eq!(resolved.date_utc, Some(1_720_108_800));
     }
 
+    #[test]
+    fn ifd0_modify_date_is_a_last_resort() {
+        // Scans / edits with only EXIF DateTime (exiftool: IFD0:ModifyDate)
+        // resolve from it as a camera wall clock, like the kamadak path.
+        let resolved = resolve_exiftool_payload(
+            &tags(&[
+                ("IFD0", "ModifyDate", "2009:03:04 14:55:48"),
+                ("ExifIFD", "DateTimeOriginal", ""),
+            ]),
+            &ExiftoolEvidence {
+                offset_time: Some(-300),
+                ..Default::default()
+            },
+        )
+        .unwrap();
+        assert_eq!(resolved.source, DateSource::Exiftool);
+        assert_eq!(resolved.date_utc, Some(1_236_196_548));
+        assert_eq!(resolved.camera_offset_minutes, Some(-300));
+
+        // Any capture tag outranks it.
+        let resolved = resolve_exiftool_payload(
+            &tags(&[
+                ("IFD0", "ModifyDate", "2022:11:08 15:18:20"),
+                ("ExifIFD", "CreateDate", "2022:11:01 09:00:00"),
+            ]),
+            &ExiftoolEvidence {
+                offset_time: Some(-240),
+                ..Default::default()
+            },
+        )
+        .unwrap();
+        assert_eq!(resolved.date_utc, Some(1_667_307_600));
+    }
+
+    #[test]
+    fn non_exif_modify_date_is_ignored() {
+        // XMP / QuickTime ModifyDate is an edit or mux time, not capture.
+        for group in ["XMP-xmp", "QuickTime"] {
+            assert!(
+                resolve_exiftool_payload(
+                    &tags(&[(group, "ModifyDate", "2024:01:01 00:00:00")]),
+                    &ExiftoolEvidence::default(),
+                )
+                .is_none(),
+                "{group}"
+            );
+        }
+    }
+
     #[test]
     fn zero_date_falls_through_to_next_tag() {
         // exiftool emits "0000:00:00 00:00:00" for missing-but-allocated