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/utils.rs

@@ -27,7 +27,7 @@ where
         Err(e) => e,
     };
     for attempt in 1..=max_retries {
-        let base = Duration::from_millis(100).saturating_mul(1_u32.pow(attempt - 1));
+        let base = Duration::from_millis(100).saturating_mul(2_u32.pow(attempt - 1));
         let jitter_range = base.as_millis() as f64 * 0.25;
         let jitter = rand::thread_rng().gen_range(-jitter_range..=jitter_range) as u128;
         let delay = base.as_millis() as i128 + jitter as i128;
@@ -92,7 +92,7 @@ where
             );
             break;
         }
-        let base = Duration::from_millis(100).saturating_mul(1_u32.pow(attempt - 1));
+        let base = Duration::from_millis(100).saturating_mul(2_u32.pow(attempt - 1));
         let jitter_range = base.as_millis() as f64 * 0.25;
         let jitter = rand::thread_rng().gen_range(-jitter_range..=jitter_range) as u128;
         let delay = base.as_millis() as i128 + jitter as i128;