chore: resolve all clippy warnings and formatting

- Replace impl ToString with impl Display for InsightJobStatus and
  InsightGenerationType
- Rename from_str → parse to avoid confusion with std::str::FromStr
- Collapse nested if statements (handlers, insight_chat, insight_generator,
  image handlers)
- Use is_multiple_of() instead of manual modulo checks
- Suppress deprecated diesel::dsl::count_distinct (no drop-in replacement
  available in current Diesel version)
- Scope MutexGuard in synthesize_merge to drop before await
- Allow dead_code on generate_no_think, enumerate_indexable_files,
  total_deleted (intended for future use)
- Allow type_complexity on Diesel query result tuples

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/ai/insight_chat.rs

@@ -1022,10 +1022,10 @@ impl InsightChatService {
         );
         let system_msg = ChatMessage::system(system_content);
         let mut user_msg = ChatMessage::user(req.user_message.clone());
-        if backend.images_inline {
-            if let Some(ref img) = image_base64 {
-                user_msg.images = Some(vec![img.clone()]);
-            }
+        if backend.images_inline
+            && let Some(ref img) = image_base64
+        {
+            user_msg.images = Some(vec![img.clone()]);
         }
         let mut messages = vec![system_msg, user_msg];