fix(dates): resume cursor so unconvertible rows can't stall the #118 drain

Review follow-up: rows that exist but can't be faithfully resolved spent
the cap and were re-collected first every tick. Carry a per-library
cursor across ticks (wrapping at end of table), share the cap parser
between the two date drains, and name the conversion tuple.

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

src/watcher.rs

@@ -349,6 +349,9 @@ pub fn watch_files(
         // Last-known per-library remaining count behind the
         // `imageserver_date_backfill_remaining` gauge.
         let mut utc_backfill_remaining: HashMap<i32, i64> = HashMap::new();
+        // Per-library resume cursor for the true-instant drain; in-memory,
+        // so a restart resumes from 0 (harmless — the drain is idempotent).
+        let mut utc_backfill_cursors: HashMap<i32, i32> = HashMap::new();
 
         let mut last_quick_scan = SystemTime::now();
         // Initialize to UNIX_EPOCH so the *first* tick is treated as a
@@ -530,6 +533,7 @@ pub fn watch_files(
                         &exif_dao,
                         &precomputed_reel_dao,
                         &display_tz,
+                        utc_backfill_cursors.entry(lib.id).or_insert(0),
                     );
                     backfill::refresh_utc_backfill_progress(
                         &context,