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/ollama.rs

@@ -424,10 +424,7 @@ impl OllamaClient {
         self.generate_with_images(prompt, system, None).await
     }
 
-    /// Variant of `generate` that sets Ollama's top-level `think: false`.
-    /// Used by latency-sensitive callers like the rerank pass, where the
-    /// task has nothing to reason about and chain-of-thought tokens are
-    /// wasted wall time. Server-side no-op on non-reasoning models.
+    #[allow(dead_code)]
     pub async fn generate_no_think(&self, prompt: &str, system: Option<&str>) -> Result<String> {
         self.generate_with_options(prompt, system, None, Some(false))
             .await