refactor(file|folder): separate etag and blob_hash

src/application/adapters/webdav_adapter.rs

@@ -444,9 +444,11 @@ impl WebDavAdapter {
         xml_writer.write_event(Event::Text(BytesText::new(&modified_at.to_rfc2822())))?;
         xml_writer.write_event(Event::End(BytesEnd::new("D:getlastmodified")))?;
 
-        // Other standard properties
+        // ETag — routes through `FolderDto::etag` (= `Folder::etag()`)
+        // so every WebDAV emitter and HEAD response agree on a single
+        // value for the same folder.
         xml_writer.write_event(Event::Start(BytesStart::new("D:getetag")))?;
-        xml_writer.write_event(Event::Text(BytesText::new(&format!("\"{}\"", folder.id))))?;
+        xml_writer.write_event(Event::Text(BytesText::new(&format!("\"{}\"", folder.etag))))?;
         xml_writer.write_event(Event::End(BytesEnd::new("D:getetag")))?;
 
         // Content length (0 for directories)
@@ -505,9 +507,11 @@ impl WebDavAdapter {
         xml_writer.write_event(Event::Text(BytesText::new(&modified_at.to_rfc2822())))?;
         xml_writer.write_event(Event::End(BytesEnd::new("D:getlastmodified")))?;
 
-        // ETag
+        // ETag — routes through `FileDto::etag` (= `File::etag()`) so
+        // PROPFIND, GET, HEAD, PUT-response, and MOVE all emit
+        // byte-identical values for the same file.
         xml_writer.write_event(Event::Start(BytesStart::new("D:getetag")))?;
-        xml_writer.write_event(Event::Text(BytesText::new(&format!("\"{}\"", file.id))))?;
+        xml_writer.write_event(Event::Text(BytesText::new(&format!("\"{}\"", file.etag))))?;
         xml_writer.write_event(Event::End(BytesEnd::new("D:getetag")))?;
 
         Ok(())
@@ -589,7 +593,7 @@ impl WebDavAdapter {
                         xml_writer.write_event(Event::Start(BytesStart::new("D:getetag")))?;
                         xml_writer.write_event(Event::Text(BytesText::new(&format!(
                             "\"{}\"",
-                            folder.id
+                            folder.etag
                         ))))?;
                         xml_writer.write_event(Event::End(BytesEnd::new("D:getetag")))?;
                     }
@@ -685,7 +689,7 @@ impl WebDavAdapter {
                         xml_writer.write_event(Event::Start(BytesStart::new("D:getetag")))?;
                         xml_writer.write_event(Event::Text(BytesText::new(&format!(
                             "\"{}\"",
-                            file.id
+                            file.etag
                         ))))?;
                         xml_writer.write_event(Event::End(BytesEnd::new("D:getetag")))?;
                     }

src/application/dtos/file_dto.rs

@@ -62,14 +62,32 @@ pub struct FileDto {
     #[serde(skip_serializing_if = "Option::is_none")]
     pub sort_date: Option<u64>,
 
-    /// Content-addressable ETag (= blob_hash). Changes on every content write.
-    /// Used for WebDAV/Nextcloud ETag headers. Omitted from REST API JSON.
-    #[serde(skip)]
+    /// Raw BLAKE3 content hash. Populated from `File::content_hash()`.
+    /// Exposed in REST JSON so API consumers can use it for
+    /// content-addressable URLs, dedup verification, and integrity
+    /// audits. Distinct from `etag` (which is an HTTP-only cache
+    /// token whose formula may grow to include `modified_at` etc.).
+    pub content_hash: String,
+
+    /// Opaque HTTP ETag. Populated from `File::etag()`. Used by
+    /// WebDAV/NextCloud handlers when emitting `ETag` headers and
+    /// also exposed in REST JSON so frontends can pass it back
+    /// through `If-Match` / `If-None-Match` on download / mutation
+    /// endpoints without a separate HEAD round-trip.
     pub etag: String,
 }
 
 impl From<File> for FileDto {
     fn from(file: File) -> Self {
+        // Compute the HTTP ETag BEFORE consuming the entity — once
+        // `File::etag()` folds modified_at (and possibly more) into
+        // the formula, this becomes more than a string clone and
+        // must run against a live entity, not against
+        // already-extracted parts. content_hash is just the blob
+        // hash; etag is the cache token derived from it.
+        let etag = file.etag().to_string();
+        let content_hash = file.content_hash().to_string();
+
         // Consume the entity by moving all fields — zero heap allocations
         // for id, name, path, folder_id, owner_id (previously 5× .to_string()).
         let parts = file.into_parts();
@@ -95,7 +113,8 @@ impl From<File> for FileDto {
             size_formatted,
             owner_id: parts.owner_id.map(|u| u.to_string()),
             sort_date: None,
-            etag: parts.etag,
+            content_hash,
+            etag,
         }
     }
 }
@@ -150,6 +169,7 @@ impl FileDto {
             category: Arc::from("Document"),
             size_formatted: "0 Bytes".to_string(),
             owner_id: None,
+            content_hash: String::new(),
             etag: String::new(),
             sort_date: None,
         }

src/application/dtos/folder_dto.rs

@@ -73,11 +73,19 @@ pub struct FolderDto {
     /// Human-readable category (always "Folder")
     #[schema(value_type = String)]
     pub category: Arc<str>,
+
+    /// Opaque ETag for HTTP responses. Populated from `Folder::etag()`
+    /// at conversion time so every WebDAV / NextCloud handler emits
+    /// the same value, and exposed in REST JSON so the frontend can
+    /// pass it back through `If-Match` on rename / move endpoints
+    /// without a separate HEAD round-trip.
+    pub etag: String,
 }
 
 impl From<Folder> for FolderDto {
     fn from(folder: Folder) -> Self {
         let is_root = folder.parent_id().is_none();
+        let etag = folder.etag().to_string();
 
         Self {
             id: folder.id().to_string(),
@@ -91,6 +99,7 @@ impl From<Folder> for FolderDto {
             icon_class: Arc::from("fas fa-folder"),
             icon_special_class: Arc::from("folder-icon"),
             category: Arc::from("Folder"),
+            etag,
         }
     }
 }
@@ -141,6 +150,7 @@ impl FolderDto {
             icon_class: Arc::from("fas fa-folder"),
             icon_special_class: Arc::from("folder-icon"),
             category: Arc::from("Folder"),
+            etag: String::new(),
         }
     }
 }

src/application/services/trash_service.rs

@@ -809,8 +809,10 @@ fn build_trash_cursor(row: &TrashResourceRow, order_by: &str, reverse: bool) ->
 fn row_to_item_dto(row: TrashResourceRow) -> TrashResourceItemDto {
     let path = row.path.clone().unwrap_or_default();
     if row.resource_type == "folder" {
+        let resource_id = row.resource_id.to_string();
         let dto = FolderDto {
-            id: row.resource_id.to_string(),
+            etag: resource_id.clone(),
+            id: resource_id,
             name: row.name.clone(),
             path,
             parent_id: row.parent_id.map(|u| u.to_string()),
@@ -834,6 +836,8 @@ fn row_to_item_dto(row: TrashResourceRow) -> TrashResourceItemDto {
             .as_deref()
             .unwrap_or("application/octet-stream");
         let size_bytes = row.size.max(0) as u64;
+        // Trash listing row doesn't carry blob_hash either; trashed
+        // items aren't ETag-conditional in the UI.
         let dto = FileDto {
             id: row.resource_id.to_string(),
             name: row.name.clone(),
@@ -849,6 +853,7 @@ fn row_to_item_dto(row: TrashResourceRow) -> TrashResourceItemDto {
             size_formatted: format_file_size(size_bytes),
             owner_id: Some(row.owner_id.to_string()),
             sort_date: None,
+            content_hash: String::new(),
             etag: String::new(),
         };
         TrashResourceItemDto {