feat(PBI-59): Sprint.code (SP-N sequentieel per product) (#153)

Voegt een verplicht code-veld toe aan Sprint, sequentieel per product
(consistent met PBI-N, ST-NNN, T-N).

- **Schema** — `Sprint.code String @db.VarChar(30)` + `@@unique([product_id, code])`
- **Migratie** — voegt kolom toe als nullable, backfillt bestaande sprints
  via `ROW_NUMBER() OVER (PARTITION BY product_id ORDER BY created_at)`
  als `SP-N`, en zet daarna NOT NULL + UNIQUE.
- **Generator** — `generateNextSprintCode(productId)` in lib/code-server.ts
  volgt het patroon van story/pbi/task; createSprintAction gebruikt
  `createWithCodeRetry` voor race-bescherming.
- **Seed** — sprint-counter per product (`SP-1`, `SP-2`, ...).

Zichtbaar in:
- Sprint-header (`Product › Sprint actief · SP-3`)
- JobCard + JobDetailPane voor SPRINT_IMPLEMENTATION jobs
- Insights: VelocityChart x-axis (compacter dan goal-truncated),
  AlignmentTrend tooltip, SprintInfoStrip
- actions/jobs-page.ts: `sprintCode` is weer een echte code i.p.v. null

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

__tests__/actions/sprint-dates.test.ts

@@ -16,6 +16,7 @@ vi.mock('@/lib/prisma', () => ({
   prisma: {
     sprint: {
       findFirst: vi.fn(),
+      findMany: vi.fn(),
       create: vi.fn(),
       update: vi.fn(),
     },
@@ -25,7 +26,7 @@ vi.mock('@/lib/prisma', () => ({
 import { prisma } from '@/lib/prisma'
 import { createSprintAction, updateSprintDatesAction } from '@/actions/sprints'
 
-const mockSprint = prisma as unknown as { sprint: { findFirst: ReturnType<typeof vi.fn>; create: ReturnType<typeof vi.fn>; update: ReturnType<typeof vi.fn> } }
+const mockSprint = prisma as unknown as { sprint: { findFirst: ReturnType<typeof vi.fn>; findMany: ReturnType<typeof vi.fn>; create: ReturnType<typeof vi.fn>; update: ReturnType<typeof vi.fn> } }
 
 function makeFormData(data: Record<string, string | null>) {
   const fd = new FormData()
@@ -39,6 +40,7 @@ describe('createSprintAction — date validation', () => {
   beforeEach(() => {
     vi.clearAllMocks()
     mockSprint.sprint.findFirst.mockResolvedValue(null)
+    mockSprint.sprint.findMany.mockResolvedValue([])
     mockSprint.sprint.create.mockResolvedValue({ id: 'sprint-1' })
   })