backfill_date_taken: surface the actual diesel error in warnings

The DAO swallowed every diesel::update failure as a flat
`anyhow!("Update error")`, then trace_db_call further reduced it to
`DbError { kind: UpdateError }`. Operators saw "update failed for lib
2 Snapchat/foo.mp4: DbError { kind: UpdateError }" with no clue why
(constraint violation? type mismatch? row vanished mid-flight? DB
locked?).

Two changes:
- Preserve the diesel error in the anyhow chain along with the input
  params (lib, rel_path, date_taken, source) so the cause is visible.
- Log the chain at warn-level inside the DAO before the trace wrapper
  collapses it to DbErrorKind::UpdateError, so the warning at the
  call site finally has something diagnosable next to it.
- Treat zero-row updates as a debug-level "row likely retired by the
  missing-file scan" rather than a hard failure — that case is benign
  and shouldn't poison the drain's error tally.

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

src/database/mod.rs

@@ -1198,17 +1198,52 @@ impl ExifDao for SqliteExifDao {
 
             let mut connection = self.connection.lock().expect("Unable to get ExifDao");
 
-            diesel::update(
+            let result = diesel::update(
                 image_exif
                     .filter(library_id.eq(library_id_val))
                     .filter(rel_path.eq(rel_path_val)),
             )
             .set((date_taken.eq(date_taken_val), date_taken_source.eq(source)))
-            .execute(connection.deref_mut())
+            .execute(connection.deref_mut());
-            .map(|_| ())
+
-            .map_err(|_| anyhow::anyhow!("Update error"))
+            match result {
+                Ok(rows) => {
+                    // Surface zero-row updates as a warning rather than a
+                    // silent success. They mean the (library_id, rel_path)
+                    // row was deleted between the `get_rows_needing_date_
+                    // backfill` query and this update — rare but possible
+                    // when the file watcher is racing the drain. The drain
+                    // shouldn't treat that as a hard error, so still
+                    // return Ok(()).
+                    if rows == 0 {
+                        log::debug!(
+                            "backfill_date_taken: 0 rows matched lib={} {} \
+                             (row likely retired by missing-file scan)",
+                            library_id_val,
+                            rel_path_val
+                        );
+                    }
+                    Ok(())
+                }
+                // Preserve the diesel error in the chain so warnings at
+                // the call site can articulate the cause (a flat "Update
+                // error" was useless for triage).
+                Err(e) => Err(anyhow::anyhow!(
+                    "diesel update failed (lib={}, rel_path={}, date_taken={}, source={}): {}",
+                    library_id_val,
+                    rel_path_val,
+                    date_taken_val,
+                    source,
+                    e
+                )),
+            }
+        })
+        .map_err(|e| {
+            // Log before the anyhow message gets stripped by the
+            // DbError-only return type.
+            log::warn!("backfill_date_taken: {}", e);
+            DbError::new(DbErrorKind::UpdateError)
         })
-        .map_err(|_| DbError::new(DbErrorKind::UpdateError))
     }
 
     fn set_manual_date_taken(