feat: phase 3 — history endpoints (start, entries, complete)

backend/prisma/schema.prisma

@@ -88,6 +88,7 @@ model Program {
 
   exercises ProgramExercise[]
   groups    GroupProgram[]
+  histories History[]
 
   @@map("programs")
 }
@@ -111,12 +112,15 @@ model ProgramExercise {
 // ─── History ─────────────────────────────────────────────────────────────────
 
 model History {
-  id      String   @id @default(uuid())
-  date    DateTime @default(now())
-  notes   String?
+  id          String    @id @default(uuid())
+  date        DateTime  @default(now())
+  completedAt DateTime?
+  notes       String?
 
-  userId String
-  user   User   @relation(fields: [userId], references: [id], onDelete: Cascade)
+  userId    String
+  user      User    @relation(fields: [userId], references: [id], onDelete: Cascade)
+  programId String
+  program   Program @relation(fields: [programId], references: [id], onDelete: Cascade)
 
   entries HistoryEntry[]