fix(webdav): make PATCH's concurrency guard a real compare-and-swap

The app-level ETag re-check before the write still left a gap between the check and the actual UPDATE for a concurrent writer to land in.
Push the check into the write path itself: swap_blob_hash now takes an expected_hash and only applies the SET under the same FOR UPDATE row lock it already held, closing the race instead of just narrowing it. Adds ErrorKind::PreconditionFailed (412) for the CAS-miss path; PUT/WOPI/chunked-upload keep blind-overwrite semantics by passing None

src/application/services/trash_service_test.rs

@@ -589,6 +589,7 @@ impl FileWritePort for MockFileRepository {
         _size: u64,
         _modified_at: Option<i64>,
         _caller_id: Uuid,
+        _expected_hash: Option<&str>,
     ) -> std::result::Result<(String, i64), DomainError> {
         Ok((String::new(), 0))
     }