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/file_scan.rs

@@ -53,6 +53,7 @@ pub fn walk_library_files(base_path: &Path, excluded_dirs: &[String]) -> Vec<Dir
 /// used by the watcher's quick-scan tick to skip the long tail. Files
 /// whose metadata can't be read are kept; the caller's batch EXIF lookup
 /// dedups against existing rows.
+#[allow(dead_code)]
 pub fn enumerate_indexable_files(
     base_path: &Path,
     excluded_dirs: &[String],