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/bin/probe_clip_search.rs

@@ -219,7 +219,7 @@ async fn main() -> anyhow::Result<()> {
                     }
                     let sim = dot(&vec, &query_vec);
                     scores.push((sim, rel_path.clone()));
-                    if encoded % 10 == 0 {
+                    if encoded.is_multiple_of(10) {
                         info!(
                             "progress: {} encoded, {:.1}s elapsed",
                             encoded,