feat: store matching metadata on session file registration

Each saved file now records three additional fields:
  matched_identifier    — the literal animal_id_string or animal_name
                          substring that matched in the filename
  animal_id_string_snap — snapshot of animal.animal_id_string at
                          registration time
  animal_name_snap      — snapshot of animal.animal_name at
                          registration time

This lets future consumers (scripts, exports, re-matching) identify
the subject from the file record alone, even if animal data changes.
The matched identifier is also shown in the drop preview UI (↳ 2852).

Tests: 25 total (+2 for new payload fields).

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

frontend/tests/FileDropZone.test.jsx

@@ -249,8 +249,8 @@ describe('remove files before saving', () => {
 // ── Saving ─────────────────────────────────────────────────────────────────────
 
 describe('saving metadata', () => {
-  it('calls sessionFilesApi.create with correct payload for each subject', async () => {
-    const file = makeFile(`${DATE}_2852_trials.csv`); // size = 1 byte (content 'x')
+  it('calls sessionFilesApi.create with correct payload including matching metadata', async () => {
+    const file = makeFile(`${DATE}_2852_trials.csv`);
     const savedFile = makeSavedFile('f1', 's-a1', file.name, String(file.size));
     sessionFilesApi.create.mockResolvedValue([savedFile]);
 
@@ -264,9 +264,34 @@ describe('saving metadata', () => {
         's-a1',
         expect.arrayContaining([
           expect.objectContaining({
-            filename:  `${DATE}_2852_trials.csv`,
-            file_size: file.size,
-            file_type: 'CSV',
+            filename:              `${DATE}_2852_trials.csv`,
+            file_size:             file.size,
+            file_type:             'CSV',
+            matched_identifier:    '2852',
+            animal_id_string_snap: '2852',
+            animal_name_snap:      'Rat 2852',
+          }),
+        ])
+      )
+    );
+  });
+
+  it('sets matched_identifier to animal_name when file matched by name', async () => {
+    const animal = { id: 'b1', animal_name: 'Rat A', animal_id_string: null };
+    const row    = { animal, status: { id: 's-b1', animal_id: 'b1', date: `${DATE}T00:00:00.000Z` } };
+    sessionFilesApi.create.mockResolvedValue([]);
+    render(<FileDropZone rows={[row]} date={DATE} />);
+    dropFiles([makeFile(`${DATE}_Rat_A_trials.csv`)]);
+    await waitFor(() => screen.getByTestId('save-btn'));
+    fireEvent.click(screen.getByTestId('save-btn'));
+    await waitFor(() =>
+      expect(sessionFilesApi.create).toHaveBeenCalledWith(
+        's-b1',
+        expect.arrayContaining([
+          expect.objectContaining({
+            matched_identifier:    'Rat A',
+            animal_id_string_snap: null,
+            animal_name_snap:      'Rat A',
           }),
         ])
       )