feat(ST-1201): add Sprint start_date/end_date + claude_jobs index migration

- Sprint model: optionele start_date en end_date (DATE) voor burndown x-as
- CREATE INDEX claude_jobs(status, finished_at) voor agent-throughput-queries
- Bestaande sprints houden NULL; burndown skipt die

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

prisma/schema.prisma

@@ -223,6 +223,8 @@ model Sprint {
   product_id   String
   sprint_goal  String
   status       SprintStatus @default(ACTIVE)
+  start_date   DateTime?    @db.Date
+  end_date     DateTime?    @db.Date
   created_at   DateTime     @default(now())
   completed_at DateTime?
   stories      Story[]