Dates: true_instant on metadata responses for instant matching

The mobile viewer anchored its "messages around this photo" window on
the legacy date_taken (a wall clock stored as UTC for EXIF rows), while
SMS-API dates are true instants: the window was off by the photo's UTC
offset (4-5 h for photos taken at home; ImageApi #113 user story 3).
Verified SMS-API side: importers store backup epochs as UTC, and 40/40
sampled rows equal their source epochs exactly.

`/image/metadata` (+ GPS / date mutation responses) now carries
`true_instant` from `date_view::file_true_instant`: `row_true_instant`,
else the filename-class instant, else the earliest fs time. Server-side
for the same reason as `wall_clock`: the drain's permanent floor rows
have no `date_utc`. `set_capture_dates` fills both fields at all four
response sites.

dev/e2e: the manifest records each fixture's expected instant (the wall
read in its GPS zone, else America/New_York) and `e2e.sh check` asserts
it (97/97).

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

src/data/mod.rs

@@ -263,6 +263,10 @@ pub struct MetadataResponse {
     /// (`2018-09-29T14:00:00`) — `date_view::file_wall_clock`. Render it
     /// verbatim; never zone-convert it (ADR-0001).
     pub wall_clock: Option<String>,
+    /// True instant of capture (unix seconds) — `date_view::file_true_instant`.
+    /// For instant-to-instant matching (SMS / location windows); never
+    /// render it, render `wall_clock`.
+    pub true_instant: Option<i64>,
 }
 
 impl From<fs::Metadata> for MetadataResponse {
@@ -281,7 +285,8 @@ impl From<fs::Metadata> for MetadataResponse {
             filename_date: None, // Will be set in endpoint handler
             library_id: None,
             library_name: None,
-            wall_clock: None, // Set in the endpoint handler
+            wall_clock: None,   // Set in the endpoint handler
+            true_instant: None, // Set in the endpoint handler
         }
     }
 }

src/date_view.rs

@@ -242,6 +242,26 @@ pub fn file_wall_clock(
     wall.map(format_wall_clock)
 }
 
+/// True instant for a file that may have no `image_exif` row yet — the
+/// [`file_wall_clock`] precedence (row, then filename-class date, then
+/// earliest fs time). The anchor for instant-to-instant matching (the
+/// "messages around this photo" window), never for display.
+pub fn file_true_instant(
+    row: Option<&ImageExif>,
+    rel_path: &str,
+    fs_earliest: Option<i64>,
+    display_zone: Tz,
+    server_zone: Option<Tz>,
+) -> Option<i64> {
+    if let Some(instant) = row.and_then(|r| row_true_instant(r, display_zone, server_zone)) {
+        return Some(instant);
+    }
+    match crate::memories::extract_date_from_filename(rel_path) {
+        Some(fd) => filename_true_instant(&fd, server_zone),
+        None => fs_earliest,
+    }
+}
+
 /// Render a unix instant as a naive wall in `zone`.
 fn render_in_zone_opt(instant: i64, zone: Tz) -> Option<NaiveDateTime> {
     zone.timestamp_opt(instant, 0)
@@ -720,4 +740,43 @@ mod tests {
             None
         );
     }
+
+    #[test]
+    fn file_true_instant_uses_the_row_then_filename_then_fs_time() {
+        // Row: the converted instant, not the wall (Tokyo 14:00 = 05:00Z).
+        let r = row(
+            "otd.jpg",
+            Some(utc_epoch(2018, 9, 29, 14, 0)),
+            Some("exif"),
+            Some(utc_epoch(2018, 9, 29, 5, 0)),
+            Some(540),
+        );
+        assert_eq!(
+            file_true_instant(Some(&r), "otd.jpg", Some(0), ny(), Some(ny())),
+            Some(utc_epoch(2018, 9, 29, 5, 0))
+        );
+        // No row: a wall-pattern filename is read in the server zone
+        // (10:15 EDT = 14:15Z).
+        assert_eq!(
+            file_true_instant(
+                None,
+                "Screenshots/Screenshot_20220315-101500.png",
+                Some(0),
+                ny(),
+                Some(ny())
+            ),
+            Some(utc_epoch(2022, 3, 15, 14, 15))
+        );
+        // No row, no filename date: fs time is already an instant.
+        assert_eq!(
+            file_true_instant(
+                None,
+                "Undated/scan.jpg",
+                Some(1_425_229_200),
+                ny(),
+                Some(ny())
+            ),
+            Some(1_425_229_200)
+        );
+    }
 }

src/handlers/image.rs

@@ -528,12 +528,11 @@ pub async fn get_file_metadata(
                 .lock()
                 .ok()
                 .and_then(|mut dao| dao.get_exif(&path.path).ok().flatten());
-            response.wall_clock = crate::date_view::file_wall_clock(
+            set_capture_dates(
+                &mut response,
                 row.as_ref(),
                 &path.path,
-                earliest_fs_time(&response),
                 crate::date_view::owner_display_zone(&app_state.user_ai_prefs_dao),
-                crate::camera_tz::server_zone(),
             );
             response.exif = row.map(Into::into);
 
@@ -714,14 +713,14 @@ pub async fn set_image_gps(
                     library_id: None,
                     library_name: None,
                     wall_clock: None,
+                    true_instant: None,
                 },
             };
-            response.wall_clock = crate::date_view::file_wall_clock(
+            set_capture_dates(
+                &mut response,
                 Some(&row),
                 &normalized_path,
-                earliest_fs_time(&response),
                 crate::date_view::owner_display_zone(&app_state.user_ai_prefs_dao),
-                crate::camera_tz::server_zone(),
             );
             response.exif = Some(row.into());
             response.library_id = Some(resolved_library.id);
@@ -881,15 +880,10 @@ fn build_metadata_response_for_date_mutation(
             library_id: None,
             library_name: None,
             wall_clock: None,
+            true_instant: None,
         },
     };
-    response.wall_clock = crate::date_view::file_wall_clock(
-        Some(&exif),
-        rel_path,
-        earliest_fs_time(&response),
-        display_zone,
-        crate::camera_tz::server_zone(),
-    );
+    set_capture_dates(&mut response, Some(&exif), rel_path, display_zone);
     response.exif = Some(exif.into());
     response.library_id = Some(library.id);
     response.library_name = Some(library.name.clone());
@@ -898,6 +892,23 @@ fn build_metadata_response_for_date_mutation(
     response
 }
 
+/// Fill the capture-date fields every metadata response carries (ADR-0001):
+/// `wall_clock` to display and `true_instant` to match against. Needs the
+/// fs times already on the response (the unindexed-file fallback).
+fn set_capture_dates(
+    response: &mut MetadataResponse,
+    row: Option<&ImageExif>,
+    rel_path: &str,
+    display_zone: chrono_tz::Tz,
+) {
+    let server_zone = crate::camera_tz::server_zone();
+    let fs_earliest = earliest_fs_time(response);
+    response.wall_clock =
+        crate::date_view::file_wall_clock(row, rel_path, fs_earliest, display_zone, server_zone);
+    response.true_instant =
+        crate::date_view::file_true_instant(row, rel_path, fs_earliest, display_zone, server_zone);
+}
+
 /// Earlier of the file's created / modified times — the ingest waterfall's
 /// `fs_time` rule — for the wall-clock fallback of unindexed files.
 fn earliest_fs_time(response: &MetadataResponse) -> Option<i64> {