image_exif: add date_taken_source column

New nullable TEXT column tracks which step of the canonical-date
waterfall (kamadak-exif → exiftool → filename → fs_time) populated
`date_taken`. Lets a later per-tick drain re-resolve weak sources
(`fs_time`) once stronger ones become available, and gives the UI/debug
surface a way to answer "why does this photo show up under this date?".

Adds the column at all `InsertImageExif` construction sites with `None`
placeholders (the resolver wiring lands in a follow-up commit), and
extends the `update_exif` SET tuple so the column survives the GPS-write
re-read path. Partial index `idx_image_exif_date_backfill` is created
for the upcoming drain query.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/database/mod.rs

@@ -730,6 +730,7 @@ impl ExifDao for SqliteExifDao {
                 shutter_speed.eq(&exif_data.shutter_speed),
                 iso.eq(&exif_data.iso),
                 date_taken.eq(&exif_data.date_taken),
+                date_taken_source.eq(&exif_data.date_taken_source),
                 last_modified.eq(&exif_data.last_modified),
             ))
             .execute(connection.deref_mut())
@@ -1819,6 +1820,7 @@ mod exif_dao_tests {
                 size_bytes: None,
                 phash_64: None,
                 dhash_64: None,
+                date_taken_source: None,
             },
         )
         .expect("insert exif row");