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/prisma/schema.prisma

@@ -43,10 +43,25 @@ model DailyStatus {
   analysis_summary       Json?
   animal                 Animal          @relation(fields: [animal_id], references: [id], onDelete: Cascade)
   analyses               DailyAnalysis[]
+  session_files          SessionFile[]
 
   @@map("daily_statuses")
 }
 
+model SessionFile {
+  id              String      @id @default(uuid())
+  daily_status_id String
+  daily_status    DailyStatus @relation(fields: [daily_status_id], references: [id], onDelete: Cascade)
+  filename        String
+  file_size       BigInt
+  file_type       String?
+  last_modified   BigInt?
+  notes           String?
+  created_at      DateTime    @default(now())
+
+  @@map("session_files")
+}
+
 model DailyAnalysis {
   id                String      @id @default(uuid())
   daily_status_id   String