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>

backend/prisma/schema.prisma

@@ -49,15 +49,18 @@ model DailyStatus {
 }
 
 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())
+  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?
+  matched_identifier    String?     // the animal ID/name substring that matched in the filename
+  animal_id_string_snap String?     // snapshot of animal.animal_id_string at registration time
+  animal_name_snap      String?     // snapshot of animal.animal_name at registration time
+  notes                 String?
+  created_at            DateTime    @default(now())
 
   @@map("session_files")
 }