feat(dates): backfill drain converts mixed-convention rows to true instants (#118)

Per-tick drain (backfill_utc_dates) at the existing date-drain seam,
inside the per-library online gate. Selects date_utc IS NULL AND
date_taken IS NOT NULL (new partial index idx_image_exif_utc_backfill),
re-runs resolve_dates_batch with empty priors (one exiftool subprocess
per batch, same call shape as the #117 audit), and writes only the
new-contract triple; the legacy pair stays frozen. Capped by
DATE_BACKFILL_MAX_PER_TICK; missing-file rows are paged past without
spending the cap. Legacy manual overrides convert via
manual_date_resolution rather than being re-resolved from the file.

Day-boundary invalidation deletes the day-span precomputed reels
covering a re-dated row's old/new display-zone month-day so the nightly
pre-gen regenerates them. Progress: imageserver_date_backfill_remaining
gauge + date_backfill_remaining on GET /libraries.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/files.rs

@@ -1756,6 +1756,35 @@ mod tests {
             Ok(())
         }
 
+        fn get_rows_needing_utc_backfill(
+            &mut self,
+            _context: &opentelemetry::Context,
+            _library_id: i32,
+            _after_id: i32,
+            _limit: i64,
+        ) -> Result<Vec<crate::database::UtcBackfillCandidate>, DbError> {
+            Ok(Vec::new())
+        }
+
+        fn backfill_date_utc(
+            &mut self,
+            _context: &opentelemetry::Context,
+            _id: i32,
+            _date_utc: i64,
+            _camera_offset_minutes: Option<i32>,
+            _date_taken_tz_source: &str,
+        ) -> Result<bool, DbError> {
+            Ok(false)
+        }
+
+        fn count_rows_needing_utc_backfill(
+            &mut self,
+            _context: &opentelemetry::Context,
+            _library_id: i32,
+        ) -> Result<i64, DbError> {
+            Ok(0)
+        }
+
         fn set_manual_date_taken(
             &mut self,
             _context: &opentelemetry::Context,