fix: critical bugs from deep audit

- Fix copy_files() data loss: implement real copy_file across full stack
  (FileWritePort, FileManagementUseCase, stubs, service, repository with
  atomic CTE + dedup ref_count increment, batch_operations caller)
- Fix plaintext password in replace_default_admin: hash password via
  PasswordHasherPort before User::new()
- Fix CalendarService hardcoded user_id: unify CalendarUseCase trait with
  explicit user_id parameter on all methods, remove zombie _for_user
  duplicates and hardcoded 'current_user_id', update 20 CalDAV handler
  call sites
- Previous session: migrate DedupService to PostgreSQL (storage.blobs),
  atomic CTEs with compensation for file/folder repository operations

src/common/stubs.rs

@@ -168,6 +168,14 @@ impl FileWritePort for StubFileWritePort {
         Ok(File::default())
     }
 
+    async fn copy_file(
+        &self,
+        _file_id: &str,
+        _target_folder_id: Option<String>,
+    ) -> Result<File, DomainError> {
+        Ok(File::default())
+    }
+
     async fn rename_file(&self, _file_id: &str, _new_name: &str) -> Result<File, DomainError> {
         Ok(File::default())
     }
@@ -483,6 +491,14 @@ impl FileManagementUseCase for StubFileManagementUseCase {
         Ok(FileDto::default())
     }
 
+    async fn copy_file(
+        &self,
+        _file_id: &str,
+        _folder_id: Option<String>,
+    ) -> Result<FileDto, DomainError> {
+        Ok(FileDto::default())
+    }
+
     async fn rename_file(&self, _file_id: &str, _new_name: &str) -> Result<FileDto, DomainError> {
         Ok(FileDto::default())
     }