date_backfill: per-tick drain for unresolved date_taken rows

Adds two ExifDao methods (`get_rows_needing_date_backfill` /
`backfill_date_taken`) and a `backfill_missing_date_taken` watcher pass
that runs on every tick alongside `backfill_unhashed_backlog`.

The drain queries the partial index for rows where `date_taken IS NULL`
or `date_taken_source = 'fs_time'`, batches up to
`DATE_BACKFILL_MAX_PER_TICK` paths (default 500), and feeds them through
`date_resolver::resolve_dates_batch` — a single exiftool subprocess
covers the whole tick. Rows that newly resolve to `exiftool` /
`filename` / `fs_time` get persisted via `backfill_date_taken` (touches
only `date_taken` + `date_taken_source` so EXIF / hash / perceptual
columns survive).

`filename`-sourced rows are intentionally not re-resolved — the regex
is authoritative when it matches and re-running exiftool wouldn't
change the answer. Files that have disappeared from disk are skipped
so a ghost row doesn't loop through the drain forever; the
missing-file scan in `library_maintenance` retires those separately.

Comes with two DAO unit tests (eligibility filter + column-isolation).

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

src/files.rs

@@ -1646,6 +1646,26 @@ mod tests {
             Ok(())
         }
 
+        fn get_rows_needing_date_backfill(
+            &mut self,
+            _context: &opentelemetry::Context,
+            _library_id: i32,
+            _limit: i64,
+        ) -> Result<Vec<(i32, String)>, DbError> {
+            Ok(Vec::new())
+        }
+
+        fn backfill_date_taken(
+            &mut self,
+            _context: &opentelemetry::Context,
+            _library_id: i32,
+            _rel_path: &str,
+            _date_taken: i64,
+            _source: &str,
+        ) -> Result<(), DbError> {
+            Ok(())
+        }
+
         fn find_by_content_hash(
             &mut self,
             _context: &opentelemetry::Context,