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/main.rs

@@ -151,6 +151,12 @@ fn main() -> std::io::Result<()> {
             .registry
             .register(Box::new(thumbnails::VIDEO_GAUGE.clone()))
             .unwrap();
+        // True-instant date drain progress (#118). Refreshed per watcher
+        // tick; see `backfill::refresh_utc_backfill_progress`.
+        prometheus
+            .registry
+            .register(Box::new(backfill::DATE_BACKFILL_REMAINING.clone()))
+            .unwrap();
         // HLS readiness gauges. Updated by the watcher every full-scan
         // tick and on every `/hls/stats` request. See `hls_stats`.
         prometheus