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/api/client.js

@@ -69,6 +69,13 @@ export const systemApi = {
   info: () => api.get('/info').then((r) => r.data),
 };
 
+// ── Session Files ─────────────────────────────────────────────────────────────
+export const sessionFilesApi = {
+  list:   (statusId) => api.get(`/daily-statuses/${statusId}/files`).then((r) => r.data),
+  create: (statusId, files) => api.post(`/daily-statuses/${statusId}/files`, files).then((r) => r.data),
+  delete: (id) => api.delete(`/session-files/${id}`),
+};
+
 // ── Analyses ──────────────────────────────────────────────────────────────────
 export const analysesApi = {
   listForStatus: (statusId) => api.get(`/daily-statuses/${statusId}/analyses`).then((r) => r.data),