feat(backend): Prisma schema, Express routes, audit middleware, error handling

backend/prisma/schema.prisma

@@ -0,0 +1,52 @@
+generator client {
+  provider = "prisma-client-js"
+}
+
+datasource db {
+  provider = "postgresql"
+  url      = env("DATABASE_URL")
+}
+
+model Experiment {
+  id         String   @id @default(uuid())
+  title      String
+  created_at DateTime @default(now())
+  animals    Animal[]
+
+  @@map("experiments")
+}
+
+model Animal {
+  id               String        @id @default(uuid())
+  experiment_id    String
+  animal_id_string String
+  animal_name      String
+  experiment       Experiment    @relation(fields: [experiment_id], references: [id], onDelete: Cascade)
+  daily_statuses   DailyStatus[]
+
+  @@map("animals")
+}
+
+model DailyStatus {
+  id                     String   @id @default(uuid())
+  animal_id              String
+  date                   DateTime @db.Date
+  experiment_description String?
+  vitals                 String?
+  treatment              String?
+  notes                  String?
+  animal                 Animal   @relation(fields: [animal_id], references: [id], onDelete: Cascade)
+
+  @@map("daily_statuses")
+}
+
+model AuditLog {
+  id         String   @id @default(uuid())
+  table_name String
+  record_id  String
+  action     String
+  changes    Json
+  timestamp  DateTime @default(now())
+
+  @@map("audit_logs")
+}