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/clip_search.rs

@@ -109,7 +109,7 @@ struct SearchError {
 /// `None` on malformed bytes — those rows get skipped rather than
 /// failing the whole query.
 fn decode_embedding(bytes: &[u8]) -> Option<Vec<f32>> {
-    if bytes.is_empty() || bytes.len() % 4 != 0 {
+    if bytes.is_empty() || !bytes.len().is_multiple_of(4) {
         return None;
     }
     let mut out = Vec::with_capacity(bytes.len() / 4);