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/models.rs

@@ -63,6 +63,12 @@ pub struct InsertImageExif {
     pub phash_64: Option<i64>,
     /// 64-bit dHash (gradient). NULL for videos and decode failures.
     pub dhash_64: Option<i64>,
+    /// Which step of the canonical-date waterfall populated `date_taken`:
+    /// `"exif"` | `"exiftool"` | `"filename"` | `"fs_time"`. NULL when
+    /// `date_taken` is NULL (no source resolved it). The per-tick backfill
+    /// drain re-resolves rows whose source is `"fs_time"` once exiftool
+    /// has had a chance to run.
+    pub date_taken_source: Option<String>,
 }
 
 // Field order matches the post-migration column order in `image_exif`.
@@ -98,6 +104,8 @@ pub struct ImageExif {
     pub duplicate_of_hash: Option<String>,
     /// Unix seconds at which the resolve was committed.
     pub duplicate_decided_at: Option<i64>,
+    /// Which step of the canonical-date waterfall populated `date_taken`.
+    pub date_taken_source: Option<String>,
 }
 
 #[derive(Insertable)]