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/watcher.rs

@@ -787,10 +787,16 @@ pub fn process_new_files(
             // computed above) → exiftool fallback for videos / MakerNote /
             // QuickTime → filename regex → earliest_fs_time. Source is
             // recorded so the per-tick backfill drain can re-run weak
-            // resolutions later.
+            // resolutions later. The EXIF branch additionally runs the
+            // camera-offset cascade to produce the new-contract columns
+            // (date_utc / camera_offset_minutes / date_taken_tz_source,
+            // ADR-0001) while the legacy date_taken stays byte-identical.
             let resolved_date = date_resolver::resolve_date_taken(
                 &file_path,
-                exif_fields.as_ref().and_then(|e| e.date_taken),
+                exif_fields
+                    .as_ref()
+                    .and_then(date_resolver::exif_prior_from)
+                    .as_ref(),
             );
 
             let insert_exif = InsertImageExif {
@@ -827,6 +833,11 @@ pub fn process_new_files(
                 phash_64: perceptual.map(|h| h.phash_64),
                 dhash_64: perceptual.map(|h| h.dhash_64),
                 date_taken_source: resolved_date.map(|r| r.source.as_str().to_string()),
+                date_utc: resolved_date.and_then(|r| r.date_utc),
+                camera_offset_minutes: resolved_date.and_then(|r| r.camera_offset_minutes),
+                date_taken_tz_source: resolved_date
+                    .and_then(|r| r.tz_source)
+                    .map(|t| t.as_str().to_string()),
             };
 
             let mut dao = exif_dao.lock().expect("Unable to lock ExifDao");