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>

migrations/2026-05-06-000000_add_date_taken_source/down.sql

@@ -0,0 +1,2 @@
+DROP INDEX IF EXISTS idx_image_exif_date_backfill;
+ALTER TABLE image_exif DROP COLUMN date_taken_source;

migrations/2026-05-06-000000_add_date_taken_source/up.sql

@@ -0,0 +1,24 @@
+-- Tracks where a row's `date_taken` was sourced so the canonical-date
+-- waterfall (kamadak-exif → exiftool → filename → earliest_fs_time) is
+-- visible to debugging and to the per-tick backfill drain that re-runs
+-- weak sources once stronger ones become available (e.g. exiftool gets
+-- installed on a deploy that didn't have it). See CLAUDE.md → Memories
+-- canonical-date pipeline.
+--
+-- Values:
+--   'exif'     — kamadak-exif read DateTime/DateTimeOriginal directly
+--   'exiftool' — exiftool fallback caught a video / MakerNote / QuickTime tag
+--   'filename' — extract_date_from_filename matched a known pattern
+--   'fs_time'  — fell through to earliest_fs_time(metadata)
+--
+-- NULL when `date_taken` itself is NULL (no source resolved the date).
+ALTER TABLE image_exif ADD COLUMN date_taken_source TEXT;
+
+-- Partial index for the per-tick backfill drain: targets rows that need
+-- re-resolution (no date yet, or only the weakest source resolved it).
+-- Filename-sourced rows are intentionally excluded — the regex is
+-- authoritative when it matches and re-running exiftool wouldn't change
+-- the answer.
+CREATE INDEX idx_image_exif_date_backfill
+    ON image_exif (library_id, id)
+    WHERE date_taken IS NULL OR date_taken_source = 'fs_time';

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");

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)]

src/database/schema.rs

@@ -125,6 +125,7 @@ diesel::table! {
         dhash_64 -> Nullable<BigInt>,
         duplicate_of_hash -> Nullable<Text>,
         duplicate_decided_at -> Nullable<BigInt>,
+        date_taken_source -> Nullable<Text>,
     }
 }
 

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(),
             })
         }
 

src/main.rs

@@ -537,6 +537,8 @@ async fn set_image_gps(
         // with a usable signal; failure just leaves prior values in place.
         phash_64: perceptual_hash::compute(&full_path).map(|h| h.phash_64),
         dhash_64: perceptual_hash::compute(&full_path).map(|h| h.dhash_64),
+        // Replaced in a follow-up commit with the canonical-date resolver's output.
+        date_taken_source: None,
     };
 
     let updated = {
@@ -772,6 +774,8 @@ async fn upload_image(
                             size_bytes,
                             phash_64: perceptual.map(|h| h.phash_64),
                             dhash_64: perceptual.map(|h| h.dhash_64),
+                            // Replaced in a follow-up commit with the canonical-date resolver's output.
+                            date_taken_source: None,
                         };
 
                         if let Ok(mut dao) = exif_dao.lock() {
@@ -2410,6 +2414,8 @@ fn process_new_files(
                 size_bytes,
                 phash_64: perceptual.map(|h| h.phash_64),
                 dhash_64: perceptual.map(|h| h.dhash_64),
+                // Replaced in a follow-up commit with the canonical-date resolver's output.
+                date_taken_source: None,
             };
 
             let mut dao = exif_dao.lock().expect("Unable to lock ExifDao");