feat(11-02): wire Region/PLZ/Bundesland filter into query builder + FilterPanel

TenderQueryDto gains validated plz (@MaxLength(5)), region, and
bundesland fields. buildTenderWhere adds three conditional AND-branches:
plz startsWith, bundesland exact-match against the now-backfilled indexed
column (Pitfall 1), and region startsWith (usable independent of the
bundesland column). FilterPanel gets a PLZ input and a 16-Land Bundesland
dropdown (mirrors NUTS1_BUNDESLAND — web/api are separate packages) that
write plz/bundesland into the URL searchParams; ResultsList already
forwards the full URLSearchParams to listTenders(), so no additional
fetch wiring was needed.

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

apps/api/src/tenders/tender-query.builder.ts

@@ -68,6 +68,28 @@ export function buildTenderWhere(dto: TenderQueryDto): Prisma.TenderWhereInput {
     );
   }
 
+  // FILTER-02 / D-02: PLZ prefix match — plz is a 5-digit German postal
+  // code column; startsWith allows shorter prefixes to broaden the match.
+  if (dto.plz) {
+    AND.push({ plz: { startsWith: dto.plz } });
+  }
+
+  // FILTER-02 / D-02: Bundesland exact match against the backfilled/
+  // normalizer-derived `bundesland` column (indexed, Pitfall 1) — real
+  // hits only after the 20260721140000_tender_bundesland_backfill
+  // migration has run. Preferred over region-prefix matching once
+  // bundesland is populated.
+  if (dto.bundesland) {
+    AND.push({ bundesland: dto.bundesland });
+  }
+
+  // FILTER-02 / D-02: raw NUTS-region prefix match, independent of the
+  // bundesland column — usable even for rows whose bundesland derivation
+  // hasn't run yet, and for finer-grained region-level filtering.
+  if (dto.region) {
+    AND.push({ region: { startsWith: dto.region } });
+  }
+
   if (AND.length) where.AND = AND;
   return where;
 }