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>

backend/src/routes/sessionFiles.js

@@ -0,0 +1,77 @@
+const router = require('express').Router();
+const prisma = require('../lib/prisma');
+
+// POST /api/daily-statuses/:id/files  — register file metadata (batch)
+// Body: [{ filename, file_size, file_type, last_modified, notes }, ...]
+router.post('/daily-statuses/:id/files', async (req, res, next) => {
+  try {
+    const status = await prisma.dailyStatus.findUnique({ where: { id: req.params.id } });
+    if (!status) return res.status(404).json({ error: 'Daily status not found' });
+
+    const files = req.body;
+    if (!Array.isArray(files) || files.length === 0)
+      return res.status(422).json({ error: 'Request body must be a non-empty array of file metadata' });
+
+    if (files.length > 200)
+      return res.status(422).json({ error: 'At most 200 files per request' });
+
+    for (const f of files) {
+      if (!f.filename || typeof f.filename !== 'string' || !f.filename.trim())
+        return res.status(422).json({ error: 'Each file must have a non-empty filename' });
+      if (f.file_size == null || typeof f.file_size !== 'number' || f.file_size < 0)
+        return res.status(422).json({ error: `file_size must be a non-negative number (got: ${f.file_size})` });
+    }
+
+    const created = await prisma.$transaction(
+      files.map((f) =>
+        prisma.sessionFile.create({
+          data: {
+            daily_status_id: req.params.id,
+            filename:      f.filename.trim(),
+            file_size:     BigInt(Math.round(f.file_size)),
+            file_type:     f.file_type ?? null,
+            last_modified: f.last_modified != null ? BigInt(Math.round(f.last_modified)) : null,
+            notes:         f.notes ?? null,
+          },
+        })
+      )
+    );
+
+    res.status(201).json(created.map(serializeFile));
+  } catch (err) { next(err); }
+});
+
+// GET /api/daily-statuses/:id/files
+router.get('/daily-statuses/:id/files', async (req, res, next) => {
+  try {
+    const status = await prisma.dailyStatus.findUnique({ where: { id: req.params.id } });
+    if (!status) return res.status(404).json({ error: 'Daily status not found' });
+
+    const files = await prisma.sessionFile.findMany({
+      where: { daily_status_id: req.params.id },
+      orderBy: { created_at: 'asc' },
+    });
+    res.json(files.map(serializeFile));
+  } catch (err) { next(err); }
+});
+
+// DELETE /api/session-files/:id
+router.delete('/session-files/:id', async (req, res, next) => {
+  try {
+    const file = await prisma.sessionFile.findUnique({ where: { id: req.params.id } });
+    if (!file) return res.status(404).json({ error: 'Session file not found' });
+    await prisma.sessionFile.delete({ where: { id: req.params.id } });
+    res.status(204).send();
+  } catch (err) { next(err); }
+});
+
+// BigInt fields must be serialized to strings to survive JSON.stringify
+function serializeFile(f) {
+  return {
+    ...f,
+    file_size:     f.file_size.toString(),
+    last_modified: f.last_modified != null ? f.last_modified.toString() : null,
+  };
+}
+
+module.exports = router;