lib: idea-code generator + plan_md yaml-frontmatter parser (M12 T-494)

- lib/idea-code.ts: pure formatIdeaCode helper (client-safe, no prisma)
- lib/idea-code-server.ts: atomic nextIdeaCode via Prisma row-lock,
  accepts optional TransactionClient for combining with idea.create
- lib/idea-plan-parser.ts: parsePlanMd extracts ---yaml---/body, runs
  yaml.parse + ideaPlanMdFrontmatterSchema, returns line-info on failure;
  CRLF-tolerant
- adds yaml@^2.8.4 dependency
- 8 unit tests (parser happy/missing/yaml-error/zod-error/empty-stories/CRLF;
  formatIdeaCode pad-3 + overflow)

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

lib/idea-code-server.ts

@@ -0,0 +1,26 @@
+// Atomic per-user idea-code generator (DB-side).
+// Schema: User.idea_code_counter Int @default(0) — increment-and-return via
+// Prisma `update` (which acquires a row-lock for the duration of the
+// transaction; concurrent calls serialize). Format: "IDEA-001", "IDEA-002", …
+//
+// Concurrency: vertrouwt op Postgres row-locking binnen Prisma `update`.
+// Geen aparte $transaction nodig voor enkelvoudige update — de update is
+// atomisch op één rij. Voor combineren met een idea.create wordt
+// nextIdeaCode aangeroepen binnen de bredere $transaction van de caller.
+
+import { prisma } from '@/lib/prisma'
+import { formatIdeaCode } from '@/lib/idea-code'
+
+import type { Prisma } from '@prisma/client'
+
+export async function nextIdeaCode(
+  userId: string,
+  client: Prisma.TransactionClient | typeof prisma = prisma,
+): Promise<string> {
+  const u = await client.user.update({
+    where: { id: userId },
+    data: { idea_code_counter: { increment: 1 } },
+    select: { idea_code_counter: true },
+  })
+  return formatIdeaCode(u.idea_code_counter)
+}