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/files.rs

@@ -1508,6 +1508,7 @@ mod tests {
                 dhash_64: data.dhash_64,
                 duplicate_of_hash: None,
                 duplicate_decided_at: None,
+                date_taken_source: data.date_taken_source.clone(),
             })
         }
 
@@ -1551,6 +1552,7 @@ mod tests {
                 dhash_64: data.dhash_64,
                 duplicate_of_hash: None,
                 duplicate_decided_at: None,
+                date_taken_source: data.date_taken_source.clone(),
             })
         }