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/libraries.rs

@@ -460,6 +460,10 @@ pub struct LibraryStatus {
     #[serde(flatten)]
     pub library: Library,
     pub health: LibraryHealth,
+    /// Rows in this library not yet converted to the true-instant date
+    /// contract (`date_utc IS NULL`, ADR-0001 / ticket #118). Drains to 0
+    /// over watcher ticks; `None` when the count query failed.
+    pub date_backfill_remaining: Option<i64>,
 }
 
 #[derive(serde::Serialize)]
@@ -488,6 +492,12 @@ pub async fn list_libraries(_claims: Claims, app_state: Data<AppState>) -> impl
         .library_health
         .read()
         .unwrap_or_else(|e| e.into_inner());
+    let context = opentelemetry::Context::new();
+    let mut exif_dao = app_state
+        .insight_generator
+        .exif_dao()
+        .lock()
+        .expect("Unable to lock ExifDao");
     let libraries = live_guard
         .iter()
         .map(|lib| LibraryStatus {
@@ -496,6 +506,11 @@ pub async fn list_libraries(_claims: Claims, app_state: Data<AppState>) -> impl
                 .get(&lib.id)
                 .cloned()
                 .unwrap_or(LibraryHealth::Online),
+            // Live count off the partial index — cheap, and exact even for
+            // a stale library the watcher isn't refreshing.
+            date_backfill_remaining: exif_dao
+                .count_rows_needing_utc_backfill(&context, lib.id)
+                .ok(),
         })
         .collect();
     HttpResponse::Ok().json(LibrariesResponse {