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/database/mod.rs

@@ -2103,6 +2103,7 @@ impl ExifDao for SqliteExifDao {
         .map_err(|_| DbError::new(DbErrorKind::UpdateError))
     }
 
+    #[allow(clippy::type_complexity)]
     fn list_duplicates_exact(
         &mut self,
         context: &opentelemetry::Context,
@@ -2198,6 +2199,7 @@ impl ExifDao for SqliteExifDao {
         .map_err(|_| DbError::new(DbErrorKind::QueryError))
     }
 
+    #[allow(clippy::type_complexity)]
     fn list_perceptual_candidates(
         &mut self,
         context: &opentelemetry::Context,