fix(nextcloud): detect MIME type via magic bytes instead of trusting client header

Nextcloud app uploads sent application/octet-stream as Content-Type,
causing images to not be recognized. Now both WebDAV PUT and chunked
upload paths call refine_content_type() which detects via magic bytes,
then extension, then falls back to the client header. Also fixes
update_file() which previously hardcoded application/octet-stream.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/application/ports/file_ports.rs

@@ -63,7 +63,12 @@ pub trait FileUploadUseCase: Send + Sync + 'static {
     ) -> Result<FileDto, DomainError>;
 
     /// Updates the content of an existing file (for WebDAV)
-    async fn update_file(&self, path: &str, content: &[u8]) -> Result<(), DomainError>;
+    async fn update_file(
+        &self,
+        path: &str,
+        content: &[u8],
+        content_type: &str,
+    ) -> Result<(), DomainError>;
 
     /// Streaming update — spools body to a temp file with incremental hash,
     /// then atomically replaces the file content via dedup store.