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/knowledge.rs

@@ -803,38 +803,36 @@ async fn synthesize_merge<D: KnowledgeDao + 'static>(
             .json(serde_json::json!({"error": "source_id and target_id must differ"}));
     }
 
-    let cx = opentelemetry::Context::current();
-    let mut dao = dao.lock().expect("Unable to lock KnowledgeDao");
+    let (source, target) = {
+        let cx = opentelemetry::Context::current();
+        let mut dao = dao.lock().expect("Unable to lock KnowledgeDao");
 
-    let source = match dao.get_entity_by_id(&cx, body.source_id) {
-        Ok(Some(e)) => e,
-        Ok(None) => {
-            return HttpResponse::BadRequest()
-                .json(serde_json::json!({"error": "source entity not found"}));
-        }
-        Err(e) => {
-            log::error!("synthesize_merge source lookup: {:?}", e);
-            return HttpResponse::InternalServerError()
-                .json(serde_json::json!({"error": "Database error"}));
-        }
+        let source = match dao.get_entity_by_id(&cx, body.source_id) {
+            Ok(Some(e)) => e,
+            Ok(None) => {
+                return HttpResponse::BadRequest()
+                    .json(serde_json::json!({"error": "source entity not found"}));
+            }
+            Err(e) => {
+                log::error!("synthesize_merge source lookup: {:?}", e);
+                return HttpResponse::InternalServerError()
+                    .json(serde_json::json!({"error": "Database error"}));
+            }
+        };
+        let target = match dao.get_entity_by_id(&cx, body.target_id) {
+            Ok(Some(e)) => e,
+            Ok(None) => {
+                return HttpResponse::BadRequest()
+                    .json(serde_json::json!({"error": "target entity not found"}));
+            }
+            Err(e) => {
+                log::error!("synthesize_merge target lookup: {:?}", e);
+                return HttpResponse::InternalServerError()
+                    .json(serde_json::json!({"error": "Database error"}));
+            }
+        };
+        (source, target)
     };
-    let target = match dao.get_entity_by_id(&cx, body.target_id) {
-        Ok(Some(e)) => e,
-        Ok(None) => {
-            return HttpResponse::BadRequest()
-                .json(serde_json::json!({"error": "target entity not found"}));
-        }
-        Err(e) => {
-            log::error!("synthesize_merge target lookup: {:?}", e);
-            return HttpResponse::InternalServerError()
-                .json(serde_json::json!({"error": "Database error"}));
-        }
-    };
-
-    // Drop the DAO lock before the LLM call — the generate request
-    // is the slow part (seconds) and we don't want to block other
-    // knowledge reads while it runs.
-    drop(dao);
 
     let source_desc = if source.description.trim().is_empty() {
         "(none)".to_string()