Retry insight persistence with exponential backoff to avoid SQLite lock contention

Add retry_with_backoff utility (100ms base, ±25% jitter, 3 attempts)
and replace all 11 direct DAO calls in insight_chat.rs and
insight_generator.rs. The Mutex guard drops between retries so the
maintenance connection can release its SQLite write lock.

src/database/models.rs

@@ -192,7 +192,7 @@ pub struct ImageExif {
     pub clip_model_version: Option<String>,
 }
 
-#[derive(Insertable)]
+#[derive(Insertable, Clone)]
 #[diesel(table_name = photo_insights)]
 pub struct InsertPhotoInsight {
     pub library_id: i32,