fix(watcher): refresh planner stats on full scans to stop write-lock starvation

sqlite_stat1 was last gathered by a migration while image_exif had 402
rows and an all-NULL content_hash, so the planner treated the hash index
as non-selective and scanned it for every row in the back-ref refresh's
correlated subqueries. The face_detections pass took ~20s per watcher
tick while holding the write lock, so chat/persona-chat persistence blew
past busy_timeout and returned 500s. With fresh stats the three
refreshes drop from ~22s to ~0.1s.

- library_maintenance: add refresh_planner_stats (plain ANALYZE;
  PRAGMA optimize doesn't catch this on SQLite 3.45), run on full-scan
  ticks ahead of reconcile / back-ref refresh.
- utils: retry backoff used 1_u32.pow(..), a flat ~100ms; make it the
  documented exponential 100/200/400ms.

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

src/watcher.rs

@@ -577,6 +577,9 @@ pub fn watch_files(
             // model" for the rules.
             {
                 let mut conn = image_api::database::connect();
+                if is_full_scan {
+                    library_maintenance::refresh_planner_stats(&mut conn);
+                }
                 let _ = image_api::database::reconcile::run(&mut conn);
 
                 // Back-ref refresh: hash-keyed rows whose