fix: resolve all clippy warnings and enforce cargo fmt

- display_helpers: convert module doc-comments to regular comments,
  merge identical text/markdown + text/ branches
- search_service: replace needless range loops with slice-based pagination
- folder_repository, folder_db_repository: collapse nested if statements
- favorites_pg_repository: remove unnecessary borrow on generic arg
- file_blob_read_repository: collapse 6 nested if-let blocks
- file_blob_write_repository: collapse nested if for dedup ref decrement
- chunked_upload_service: use div_ceil(), collapse 2 nested if blocks
- folder_handler: collapse nested if-let for owner check
- webdav_handler: replace 7x io::Error::new(ErrorKind::Other, ..) with
  io::Error::other(..)
- cargo fmt applied to all files

Passes: cargo clippy --all-targets --all-features -- -D warnings

src/infrastructure/services/thumbnail_service.rs

@@ -192,11 +192,7 @@ impl ThumbnailService {
                 }
 
                 // 2. Generate thumbnail (CPU-bound, runs in spawn_blocking)
-                tracing::info!(
-                    "🎨 Generating thumbnail: {} {:?}",
-                    file_id_owned,
-                    size
-                );
+                tracing::info!("🎨 Generating thumbnail: {} {:?}", file_id_owned, size);
                 match self.generate_thumbnail(&original_owned, size).await {
                     Ok(bytes) => {
                         // Save to disk (best-effort — don't fail the request)
@@ -245,7 +241,10 @@ impl ThumbnailService {
         let max_dim = size.max_dimension();
 
         // Acquire semaphore permit — bounds peak RAM from concurrent decodes
-        let _permit = self.decode_semaphore.acquire().await
+        let _permit = self
+            .decode_semaphore
+            .acquire()
+            .await
             .map_err(|_| ThumbnailError::TaskError("Decode semaphore closed".into()))?;
 
         // Run image processing in blocking thread pool
@@ -278,9 +277,9 @@ impl ThumbnailService {
             // Adaptive filter: faster filters for smaller sizes where
             // quality difference vs Lanczos3 is imperceptible
             let filter = match size {
-                ThumbnailSize::Icon    => FilterType::Triangle,   // 150px — max speed
+                ThumbnailSize::Icon => FilterType::Triangle, // 150px — max speed
                 ThumbnailSize::Preview => FilterType::CatmullRom, // 400px — good balance
-                ThumbnailSize::Large   => FilterType::CatmullRom, // 800px — sufficient quality
+                ThumbnailSize::Large => FilterType::CatmullRom, // 800px — sufficient quality
             };
             let thumbnail = img.resize(new_width, new_height, filter);
 
@@ -312,7 +311,10 @@ impl ThumbnailService {
             let _permit = match self.decode_semaphore.acquire().await {
                 Ok(p) => p,
                 Err(_) => {
-                    tracing::warn!("Decode semaphore closed, skipping thumbnails for {}", file_id);
+                    tracing::warn!(
+                        "Decode semaphore closed, skipping thumbnails for {}",
+                        file_id
+                    );
                     return;
                 }
             };
@@ -333,8 +335,8 @@ impl ThumbnailService {
                     )));
                 }
 
-                let img = image::open(&path)
-                    .map_err(|e| ThumbnailError::ImageError(e.to_string()))?;
+                let img =
+                    image::open(&path).map_err(|e| ThumbnailError::ImageError(e.to_string()))?;
 
                 let (orig_w, orig_h) = (img.width(), img.height());
 
@@ -352,18 +354,15 @@ impl ThumbnailService {
                         };
 
                         let filter = match size {
-                            ThumbnailSize::Icon    => FilterType::Triangle,
+                            ThumbnailSize::Icon => FilterType::Triangle,
                             ThumbnailSize::Preview => FilterType::CatmullRom,
-                            ThumbnailSize::Large   => FilterType::CatmullRom,
+                            ThumbnailSize::Large => FilterType::CatmullRom,
                         };
                         let thumb = img.resize(new_w, new_h, filter);
 
                         let mut buf = Vec::new();
                         thumb
-                            .write_to(
-                                &mut std::io::Cursor::new(&mut buf),
-                                ImageFormat::WebP,
-                            )
+                            .write_to(&mut std::io::Cursor::new(&mut buf), ImageFormat::WebP)
                             .map_err(|e| ThumbnailError::ImageError(e.to_string()))?;
 
                         Ok((size, Bytes::from(buf)))
@@ -376,15 +375,11 @@ impl ThumbnailService {
             let thumbnails = match results {
                 Ok(Ok(t)) => t,
                 Ok(Err(e)) => {
-                    tracing::warn!(
-                        "Thumbnail generation failed for {}: {}", file_id, e
-                    );
+                    tracing::warn!("Thumbnail generation failed for {}: {}", file_id, e);
                     return;
                 }
                 Err(e) => {
-                    tracing::warn!(
-                        "Thumbnail task panicked for {}: {}", file_id, e
-                    );
+                    tracing::warn!("Thumbnail task panicked for {}: {}", file_id, e);
                     return;
                 }
             };