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,

src/database/models.rs

@@ -23,16 +23,8 @@ impl InsightJobStatus {
             Self::Cancelled => "cancelled",
         }
     }
-}
 
-impl ToString for InsightJobStatus {
-    fn to_string(&self) -> String {
-        self.as_str().to_string()
-    }
-}
-
-impl InsightJobStatus {
-    pub fn from_str(s: &str) -> Self {
+    pub fn parse(s: &str) -> Self {
         match s {
             "running" => Self::Running,
             "completed" => Self::Completed,
@@ -49,6 +41,12 @@ impl InsightJobStatus {
     }
 }
 
+impl std::fmt::Display for InsightJobStatus {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.write_str(self.as_str())
+    }
+}
+
 /// Type of insight generation (standard vs agentic).
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize)]
 #[serde(rename_all = "snake_case")]
@@ -66,19 +64,9 @@ impl InsightGenerationType {
     }
 }
 
-impl ToString for InsightGenerationType {
-    fn to_string(&self) -> String {
-        self.as_str().to_string()
-    }
-}
-
-impl InsightGenerationType {
-    pub fn from_str(s: &str) -> Self {
-        match s {
-            "standard" => Self::Standard,
-            "agentic" => Self::Agentic,
-            _ => Self::Standard,
-        }
+impl std::fmt::Display for InsightGenerationType {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.write_str(self.as_str())
     }
 }