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>

components/jobs/job-card.tsx

@@ -39,7 +39,8 @@ export default function JobCard({
   if (kind === 'TASK_IMPLEMENTATION') {
     titleText = taskCode && taskTitle ? `${taskCode} ${taskTitle}` : taskTitle || 'Taak'
   } else if (kind === 'SPRINT_IMPLEMENTATION') {
-    titleText = sprintGoal || (sprintCode ? `Sprint ${sprintCode}` : 'Sprint')
+    if (sprintCode && sprintGoal) titleText = `${sprintCode} ${sprintGoal}`
+    else titleText = sprintGoal || sprintCode || 'Sprint'
   } else if (kind === 'IDEA_GRILL' || kind === 'IDEA_MAKE_PLAN') {
     titleText = ideaCode && ideaTitle ? `${ideaCode} ${ideaTitle}` : ideaTitle || 'Idee'
   } else if (kind === 'PLAN_CHAT') {

components/jobs/job-detail-pane.tsx

@@ -25,8 +25,11 @@ function subjectLabel(job: JobWithRelations): { label: string; value: string } |
       if (!job.taskTitle) return null
       return { label: 'Taak', value: job.taskCode ? `${job.taskCode} ${job.taskTitle}` : job.taskTitle }
     case 'SPRINT_IMPLEMENTATION':
-      if (!job.sprintGoal) return null
-      return { label: 'Sprint', value: job.sprintGoal }
+      if (!job.sprintGoal && !job.sprintCode) return null
+      return {
+        label: 'Sprint',
+        value: job.sprintCode && job.sprintGoal ? `${job.sprintCode} ${job.sprintGoal}` : (job.sprintGoal ?? job.sprintCode ?? ''),
+      }
     case 'IDEA_GRILL':
     case 'IDEA_MAKE_PLAN':
     case 'PLAN_CHAT':