feat(dates): container video dates, filesystem branch, manual edit (#115)

The exiftool branch of the date waterfall now pulls raw strings with
group prefixes (one `exiftool -j -q -G1` subprocess per batch, no
`-d %s`) and classifies each candidate:

- zoned strings (own offset / Z) are true instants;
- naive strings from container groups (QuickTime/Track*/Apple) are
  UTC-rendered creation_time fields -> true instants (utc_direct).
  Running them through the camera wall-clock cascade was the
  "videos off by 4-5 hours" bug: it reinterpreted a UTC instant in
  the server zone;
- naive strings from photo-EXIF groups are camera wall clocks and run
  the offset cascade (OffsetTime* pairing + GPS evidence pulled in
  the same exiftool pass).

The legacy date_taken column stays byte-identical to the old
server-local interpretation (reproduced in Rust, DST-aware); the
new-contract date_utc / camera_offset_minutes / date_taken_tz_source
columns carry the true instant.

Manual date edit (POST /image/exif/date) reinterprets the
naive-as-UTC epoch the client sent as a wall clock in the user's
display zone (camera_tz::manual_date_resolution, tz_source =
'manual'), snapshots the pre-override new-contract triple next to the
existing legacy snapshot pair (3 new columns, migration
2026-09-25-000000), and revert restores both pairs.

ADR-0001 records the container-date finding; CLAUDE.md date-pipeline
section updated.

src/data/mod.rs

@@ -307,6 +307,14 @@ pub struct ExifMetadata {
     pub original_date_taken: Option<i64>,
     /// When `date_taken_source = "manual"`, the prior source.
     pub original_date_taken_source: Option<String>,
+    /// When `date_taken_source = "manual"`, the prior `date_utc` snapshot
+    /// (the new-contract triple the revert restores alongside the legacy
+    /// pair above).
+    pub original_date_utc: Option<i64>,
+    /// When `date_taken_source = "manual"`, the prior camera offset.
+    pub original_camera_offset_minutes: Option<i32>,
+    /// When `date_taken_source = "manual"`, the prior tz source.
+    pub original_date_taken_tz_source: Option<String>,
 }
 
 #[derive(Debug, Serialize)]
@@ -397,6 +405,9 @@ impl From<ImageExif> for ExifMetadata {
             date_taken_tz_source: exif.date_taken_tz_source,
             original_date_taken: exif.original_date_taken,
             original_date_taken_source: exif.original_date_taken_source,
+            original_date_utc: exif.original_date_utc,
+            original_camera_offset_minutes: exif.original_camera_offset_minutes,
+            original_date_taken_tz_source: exif.original_date_taken_tz_source,
         }
     }
 }
@@ -624,6 +635,9 @@ mod date_contract_tests {
             date_utc: Some(1_720_108_800),
             camera_offset_minutes: Some(-240),
             date_taken_tz_source: Some("offset_tag".to_string()),
+            original_date_utc: None,
+            original_camera_offset_minutes: None,
+            original_date_taken_tz_source: None,
         }
     }