feat: session file registry with drag-and-drop on day view

Adds a FileDropZone at the bottom of the day view where users can
drop all session files at once. Filenames are regex-matched to
subjects using their animal_id_string (word-boundary aware, so
"2852" won't match "28520"). Only metadata is recorded — files
are never uploaded.

Backend: new session_files table (id, daily_status_id, filename,
file_size BIGINT, file_type, last_modified, notes). Routes:
  POST /api/daily-statuses/:id/files  (batch create)
  GET  /api/daily-statuses/:id/files
  DELETE /api/session-files/:id

Frontend: FileDropZone shows per-subject match groups, unmatched
files, and a warning for subjects with no match. After saving,
registered files are displayed with delete capability.

test-files/: generate_test_files.sh creates 4 dummy files per
subject (2×bin, 2×csv) with correct naming for drag-and-drop
testing. Pre-generated for 2026-04-23 with IDs 2852/3076/3077/3078.

Tests: 19 tests covering drag state, regex matching, save payload,
partial-overlap prevention, multi-subject batching, and delete.

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

frontend/src/pages/ExperimentDayView.jsx

@@ -10,6 +10,7 @@ import Button from '../components/ui/Button';
 import Modal from '../components/ui/Modal';
 import DailyStatusForm from '../components/DailyStatusForm';
 import Alert from '../components/ui/Alert';
+import FileDropZone from '../components/FileDropZone';
 
 const BUILTIN_KEYS_SET = new Set(['experiment_description', 'vitals', 'treatment', 'notes']);
 
@@ -367,6 +368,9 @@ export default function ExperimentDayView() {
         </div>
       )}
 
+      {/* Session file drop zone */}
+      <FileDropZone rows={rows} date={date} />
+
       {/* Edit modal (full row) */}
       {editStatus && (
         <Modal