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/handlers/image.rs

@@ -133,15 +133,15 @@ pub async fn get_image(
                     }
                 });
 
-            if let Some(found) = existing {
-                if let Ok(file) = NamedFile::open(&found) {
-                    span.set_status(Status::Ok);
-                    return file
-                        .use_etag(true)
-                        .use_last_modified(true)
-                        .prefer_utf8(true)
-                        .into_response(&request);
-                }
+            if let Some(found) = existing
+                && let Ok(file) = NamedFile::open(&found)
+            {
+                span.set_status(Status::Ok);
+                return file
+                    .use_etag(true)
+                    .use_last_modified(true)
+                    .prefer_utf8(true)
+                    .into_response(&request);
             }
 
             // Cache miss — generate. Resize + JPEG-encode can take 100–500ms
@@ -231,15 +231,15 @@ pub async fn get_image(
                     }
                 });
 
-            if let Some(found) = existing {
-                if let Ok(file) = NamedFile::open(&found) {
-                    span.set_status(Status::Ok);
-                    return file
-                        .use_etag(true)
-                        .use_last_modified(true)
-                        .prefer_utf8(true)
-                        .into_response(&request);
-                }
+            if let Some(found) = existing
+                && let Ok(file) = NamedFile::open(&found)
+            {
+                span.set_status(Status::Ok);
+                return file
+                    .use_etag(true)
+                    .use_last_modified(true)
+                    .prefer_utf8(true)
+                    .into_response(&request);
             }
 
             let dest = hash_xlarge_path