feat(11-02): normalizer derives bundesland + backfill migration for existing rows

Replace the Phase-10-deferred `bundesland = null` assignment with
bundeslandFromRegion(region) so new ingests are Bundesland-filterable
immediately. Add @@index([bundesland]) for filter performance. New
handwritten migration 20260721140000_tender_bundesland_backfill backfills
the ~1671 pre-existing rows (idempotent UPDATE, only where bundesland IS
NULL AND region IS NOT NULL) — applied locally via
`docker exec tessera-ctl-db-1 psql`, resolved as applied in
_prisma_migrations, and prisma generate re-run.

Verified on the local dev DB: 933/1671 rows now have bundesland set
across all 16 Länder (738 remain NULL where region itself is NULL).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/api/src/tenders/tender-normalizer.service.spec.ts

@@ -89,6 +89,37 @@ describe('TenderNormalizerService', () => {
     expect(normalized.title).toBeTruthy();
   });
 
+  it('derives bundesland from region NUTS-1 prefix (FILTER-02, Pitfall 1)', () => {
+    const raw = loadRawNoticePair(NOTICE_WITH_EFORMS_ONLY_DEADLINE);
+
+    const normalized = service.normalize(raw);
+
+    if (normalized.region) {
+      expect(normalized.bundesland).not.toBeNull();
+    } else {
+      expect(normalized.bundesland).toBeNull();
+    }
+  });
+
+  it('bundesland is null when region is null/absent — never crashes', () => {
+    const raw = loadRawNoticePair(NOTICE_WITH_NO_DEADLINE_OR_VALUE);
+    const rawWithoutRegion: RawTenderRecord = {
+      ...raw,
+      ocdsPayload: {
+        ...(raw.ocdsPayload as Record<string, unknown>),
+        buyer: {
+          ...(raw.ocdsPayload as { buyer?: Record<string, unknown> })?.buyer,
+          address: { postalCode: '12345' },
+        },
+      },
+    };
+
+    const normalized = service.normalize(rawWithoutRegion);
+
+    expect(normalized.region).toBeNull();
+    expect(normalized.bundesland).toBeNull();
+  });
+
   it('dedupKey = ocid when present', () => {
     const raw = loadRawNoticePair(NOTICE_WITH_EFORMS_ONLY_DEADLINE);