Add issue change history and daily issue stats

Issue changes are recorded per field in the issue_change table, in the same
transaction as the issue itself. The new history module exposes:
- GET /api/issues/:issueId/history — change timeline grouped by save
- GET /api/issues/stats/daily — created, status transitions and status
  snapshot per day, with time zone support

The migration backfills history for existing issues. Setting a field to its
current value no longer bumps updatedAt or emits issue.updated.

prisma/schema.prisma

@@ -26,6 +26,7 @@ model User {
   notificationMessages     NotificationMessage[]
   comments                 Comment[]
   uploadedFiles            File[]
+  issueChanges             IssueChange[]
 
   @@unique([email])
   @@map("user")
@@ -96,6 +97,7 @@ model Issue {
   dueDate     DateTime?         @db.Timestamptz(3)
   comments    Comment[]
   attachments IssueAttachment[]
+  changes     IssueChange[]
   createdAt   DateTime          @default(now()) @db.Timestamptz(3)
   updatedAt   DateTime          @updatedAt @db.Timestamptz(3)
 
@@ -264,3 +266,29 @@ model CommentAttachment {
   @@index([fileId])
   @@map("comment_attachment")
 }
+
+/// История изменений задачи — одна строка на изменённое поле.
+/// Строки одного сохранения задачи объединены общим changeId.
+/// field: created | title | content | status | priority | assignee | dueDate
+/// (валидируется в коде, см. ISSUE_CHANGE_FIELDS). Значения — строки:
+/// priority — число, dueDate — ISO-дата, assignee — id пользователя.
+/// При создании задачи пишутся две строки: created и status null -> OPEN,
+/// поэтому статус задачи на любой момент восстанавливается по строкам status.
+/// У задач, созданных до появления истории, строк нет — в статистику они не попадают.
+model IssueChange {
+  id        Int      @id @default(autoincrement())
+  issueId   Int
+  issue     Issue    @relation(fields: [issueId], references: [id], onDelete: Cascade)
+  changeId  String
+  field     String
+  oldValue  String?
+  newValue  String?
+  actorId   String?
+  actor     User?    @relation(fields: [actorId], references: [id], onDelete: SetNull)
+  changedAt DateTime @db.Timestamptz(3)
+
+  @@index([issueId, changedAt])
+  @@index([field, changedAt])
+  @@index([actorId])
+  @@map("issue_change")
+}