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/handlers.rs

@@ -97,7 +97,7 @@ pub async fn generation_status_handler(
             Ok(Some(job)) => {
                 return HttpResponse::Ok().json(GenerationStatusResponse {
                     job_id: job.id,
-                    status: InsightJobStatus::from_str(&job.status),
+                    status: InsightJobStatus::parse(&job.status),
                     started_at: job.started_at,
                     completed_at: job.completed_at,
                     result_insight_id: job.result_insight_id,
@@ -133,7 +133,7 @@ pub async fn generation_status_handler(
             Ok(Some(job)) => {
                 return HttpResponse::Ok().json(GenerationStatusResponse {
                     job_id: job.id,
-                    status: InsightJobStatus::from_str(&job.status),
+                    status: InsightJobStatus::parse(&job.status),
                     started_at: job.started_at,
                     completed_at: job.completed_at,
                     result_insight_id: job.result_insight_id,
@@ -472,10 +472,9 @@ pub async fn generate_insight_handler(
                     if let Err(e) = dao.complete_job(&ctx, job_id, id) {
                         log::error!("Failed to mark job {} as completed: {:?}", job_id, e);
                     }
-                } else {
-                    if let Err(e) = dao.fail_job(&ctx, job_id, "generation returned no insight") {
-                        log::error!("Failed to mark job {} as failed: {:?}", job_id, e);
-                    }
+                } else if let Err(e) = dao.fail_job(&ctx, job_id, "generation returned no insight")
+                {
+                    log::error!("Failed to mark job {} as failed: {:?}", job_id, e);
                 }
             }
             Ok(Ok(Err(e))) => {