perf: use Arc<str> for repetitive DTO fields to eliminate clone allocations

Replace String with Arc<str> for fields that contain repeated static
values (mime_type, icon_class, icon_special_class, category) in
FileDto, FolderDto, and OptimizedFileContent.

These fields are computed from ~40 static lookup tables and cloned
on every request. With Arc<str>, clone becomes O(1) atomic increment
instead of O(n) heap allocation — saving thousands of allocations/s
under load.

Fields kept as String: id, name, path, folder_id, owner_id
(unique per item, rarely cloned).

Zero API impact — serde serializes Arc<str> identically to String.

https://claude.ai/code/session_01EbAFEfyJNLRmJHmmYDX3Tt

src/application/dtos/file_dto.rs

@@ -1,3 +1,5 @@
+use std::sync::Arc;
+
 use crate::domain::entities::file::File;
 use serde::{Deserialize, Serialize};
 
@@ -20,8 +22,9 @@ pub struct FileDto {
     /// Size in bytes
     pub size: u64,
 
-    /// MIME type
-    pub mime_type: String,
+    /// MIME type — `Arc<str>` because MIME values repeat across files
+    /// and DTOs are cloned on every request (clone is O(1) atomic increment).
+    pub mime_type: Arc<str>,
 
     /// Parent folder ID
     pub folder_id: Option<String>,
@@ -32,15 +35,15 @@ pub struct FileDto {
     /// Last modification timestamp
     pub modified_at: u64,
 
-    // ── Pre-computed display fields ──
+    // ── Pre-computed display fields (Arc<str>: values come from static tables) ──
     /// FontAwesome icon CSS class (e.g. "fas fa-file-image")
-    pub icon_class: String,
+    pub icon_class: Arc<str>,
 
     /// Extra CSS class for icon styling (e.g. "image-icon", "" when default)
-    pub icon_special_class: String,
+    pub icon_special_class: Arc<str>,
 
     /// Human-readable file category (e.g. "Image", "Document")
-    pub category: String,
+    pub category: Arc<str>,
 
     /// Human-readable formatted size (e.g. "3.27 MB")
     pub size_formatted: String,
@@ -61,13 +64,13 @@ impl From<File> for FileDto {
             name: name.to_string(),
             path: file.path_string().to_string(),
             size,
-            mime_type: mime.to_string(),
+            mime_type: Arc::from(mime),
             folder_id: file.folder_id().map(String::from),
             created_at: file.created_at(),
             modified_at: file.modified_at(),
-            icon_class: icon_class_for(name, mime).to_string(),
-            icon_special_class: icon_special_class_for(name, mime).to_string(),
-            category: category_for(name, mime).to_string(),
+            icon_class: Arc::from(icon_class_for(name, mime)),
+            icon_special_class: Arc::from(icon_special_class_for(name, mime)),
+            category: Arc::from(category_for(name, mime)),
             size_formatted: format_file_size(size),
             owner_id: file.owner_id().map(String::from),
         }
@@ -84,7 +87,7 @@ impl From<FileDto> for File {
             dto.name,
             dto.path,
             dto.size,
-            dto.mime_type,
+            dto.mime_type.to_string(),
             dto.folder_id,
             dto.created_at,
             dto.modified_at,
@@ -100,13 +103,13 @@ impl FileDto {
             name: "stub-file".to_string(),
             path: "/stub/path".to_string(),
             size: 0,
-            mime_type: "application/octet-stream".to_string(),
+            mime_type: Arc::from("application/octet-stream"),
             folder_id: None,
             created_at: 0,
             modified_at: 0,
-            icon_class: "fas fa-file".to_string(),
-            icon_special_class: String::new(),
-            category: "Document".to_string(),
+            icon_class: Arc::from("fas fa-file"),
+            icon_special_class: Arc::from(""),
+            category: Arc::from("Document"),
             size_formatted: "0 Bytes".to_string(),
             owner_id: None,
         }

src/application/dtos/folder_dto.rs

@@ -1,3 +1,5 @@
+use std::sync::Arc;
+
 use crate::domain::entities::folder::Folder;
 use serde::{Deserialize, Serialize};
 
@@ -53,15 +55,15 @@ pub struct FolderDto {
     /// Whether this is a root folder
     pub is_root: bool,
 
-    // ── Pre-computed display fields ──
+    // ── Pre-computed display fields (Arc<str>: always identical values) ──
     /// FontAwesome icon CSS class (always "fas fa-folder")
-    pub icon_class: String,
+    pub icon_class: Arc<str>,
 
     /// Extra CSS class for icon styling (always "folder-icon")
-    pub icon_special_class: String,
+    pub icon_special_class: Arc<str>,
 
     /// Human-readable category (always "Folder")
-    pub category: String,
+    pub category: Arc<str>,
 }
 
 impl From<Folder> for FolderDto {
@@ -77,9 +79,9 @@ impl From<Folder> for FolderDto {
             created_at: folder.created_at(),
             modified_at: folder.modified_at(),
             is_root,
-            icon_class: "fas fa-folder".to_string(),
-            icon_special_class: "folder-icon".to_string(),
-            category: "Folder".to_string(),
+            icon_class: Arc::from("fas fa-folder"),
+            icon_special_class: Arc::from("folder-icon"),
+            category: Arc::from("Folder"),
         }
     }
 }
@@ -112,9 +114,9 @@ impl FolderDto {
             created_at: 0,
             modified_at: 0,
             is_root: true,
-            icon_class: "fas fa-folder".to_string(),
-            icon_special_class: "folder-icon".to_string(),
-            category: "Folder".to_string(),
+            icon_class: Arc::from("fas fa-folder"),
+            icon_special_class: Arc::from("folder-icon"),
+            category: Arc::from("Folder"),
         }
     }
 }

src/application/ports/file_ports.rs

@@ -93,7 +93,7 @@ pub enum OptimizedFileContent {
     /// Small-file content (possibly transcoded / compressed) already in RAM.
     Bytes {
         data: Bytes,
-        mime_type: String,
+        mime_type: Arc<str>,
         was_transcoded: bool,
     },
     /// Memory-mapped file (10–100 MB).

src/application/services/file_retrieval_service.rs

@@ -61,7 +61,7 @@ impl FileRetrievalService {
         mime: &str,
         file_size: u64,
         accept_webp: bool,
-    ) -> Option<(Bytes, String)> {
+    ) -> Option<(Bytes, Arc<str>)> {
         if !accept_webp {
             return None;
         }
@@ -81,7 +81,7 @@ impl FileRetrievalService {
                     transcoded.len(),
                     (1.0 - transcoded.len() as f64 / content.len().max(1) as f64) * 100.0
                 );
-                Some((transcoded, webp_mime))
+                Some((transcoded, Arc::from(&*webp_mime)))
             }
             _ => None,
         }
@@ -152,7 +152,7 @@ impl FileRetrievalService {
             // Store in cache
             if let Some(cache) = &self.content_cache {
                 let etag: Arc<str> = format!("\"{}-{}\"", id, modified_at).into();
-                let ct: Arc<str> = Arc::from(&*mime_type);
+                let ct: Arc<str> = mime_type.clone();
                 cache
                     .put(id.to_string(), content_bytes.clone(), etag, ct)
                     .await;

src/application/services/search_service.rs

@@ -158,7 +158,7 @@ impl SearchService {
             name: file.name.clone(),
             path: file.path.clone(),
             size: file.size,
-            mime_type: file.mime_type.clone(),
+            mime_type: file.mime_type.to_string(),
             folder_id: file.folder_id.clone(),
             created_at: file.created_at,
             modified_at: file.modified_at,