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/prisma/schema.prisma

@@ -286,6 +286,7 @@ model Tender {
   @@index([status])
   @@index([deadlineAt])
   @@index([publishedAt])
+  @@index([bundesland]) // FILTER-02: post-backfill Bundesland-Filter-Performance
   // Deliberately NO tenant column and NO tenant index — this is global data (D-03)
 }