feat(dates): foundation for the true-instant date contract (#114)

Populate the new-contract date columns at ingest while keeping the
legacy date_taken column byte-identical (ADR-0001):

- camera_tz module: offset cascade (offset tag -> GPS zone -> server
  zone) as a pure, testable function; bundled offline TZF geometry for
  GPS -> IANA resolution; DST-correct offsets for the capture date;
  display-zone helpers (DEFAULT_DISPLAY_TZ, display_zone_for,
  display_zone_offset_minutes) for the read-side cutover.
- EXIF branch: kamadak-exif now collects OffsetTime* tags paired with
  the winning date tag; resolve_date_taken takes an ExifPrior and
  emits (date_utc, camera_offset_minutes, date_taken_tz_source) per
  branch: exif -> cascade, exiftool/fs_time -> utc_direct, filename
  -> unclassified (pending #116).
- Schema: migration adds image_exif.date_utc /
  camera_offset_minutes / date_taken_tz_source and
  user_ai_prefs.display_tz (all nullable; down.sql drops them).
- DAO + ingest: backfill_date_taken persists the triple alongside the
  legacy pair; watcher, upload, and set_image_gps call sites feed the
  cascade.
- Response surface: ExifMetadata and ExifSummary carry the triple so
  /image/metadata and /photos/exif expose it (null for unconverted
  rows); reel requests mirror display_tz into prefs.

cargo build + clippy clean; 625 lib + 702 bin tests pass (the one
failing doctest in utils.rs is pre-existing at the base commit).

src/files.rs

@@ -1281,6 +1281,9 @@ pub async fn list_exif_summary(
                     gps_latitude: r.gps_latitude.map(f64::from),
                     gps_longitude: r.gps_longitude.map(f64::from),
                     date_taken: r.date_taken,
+                    date_utc: r.date_utc,
+                    camera_offset_minutes: r.camera_offset_minutes,
+                    date_taken_tz_source: r.date_taken_tz_source,
                 })
                 .collect();
 
@@ -1513,6 +1516,9 @@ mod tests {
             original_date_taken_source: None,
             clip_embedding: None,
             clip_model_version: None,
+            date_utc: None,
+            camera_offset_minutes: None,
+            date_taken_tz_source: None,
         }
     }
 
@@ -1554,6 +1560,9 @@ mod tests {
                 original_date_taken_source: None,
                 clip_embedding: None,
                 clip_model_version: None,
+                date_utc: data.date_utc,
+                camera_offset_minutes: data.camera_offset_minutes,
+                date_taken_tz_source: data.date_taken_tz_source.clone(),
             })
         }
 
@@ -1602,6 +1611,9 @@ mod tests {
                 original_date_taken_source: None,
                 clip_embedding: None,
                 clip_model_version: None,
+                date_utc: data.date_utc,
+                camera_offset_minutes: data.camera_offset_minutes,
+                date_taken_tz_source: data.date_taken_tz_source.clone(),
             })
         }
 
@@ -1726,6 +1738,9 @@ mod tests {
             _rel_path: &str,
             _date_taken: i64,
             _source: &str,
+            _date_utc: Option<i64>,
+            _camera_offset_minutes: Option<i32>,
+            _date_taken_tz_source: Option<&str>,
         ) -> Result<(), DbError> {
             Ok(())
         }