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.spec.ts

@@ -135,6 +135,43 @@ describe('buildTenderWhere', () => {
     const where = buildTenderWhere(dto({ status: 'expired' }));
     expect(where.status).toBe('expired');
   });
+
+  it('plz set: adds a plz-startsWith constraint (FILTER-02)', () => {
+    const where = buildTenderWhere(dto({ plz: '10' }));
+
+    expect(where.AND).toEqual(
+      expect.arrayContaining([{ plz: { startsWith: '10' } }]),
+    );
+  });
+
+  it('bundesland set: adds an exact bundesland-equality constraint against the backfilled column (FILTER-02, Pitfall 1)', () => {
+    const where = buildTenderWhere(dto({ bundesland: 'Bayern' }));
+
+    expect(where.AND).toEqual(expect.arrayContaining([{ bundesland: 'Bayern' }]));
+  });
+
+  it('region set: adds a region-startsWith constraint, independent of bundesland (FILTER-02)', () => {
+    const where = buildTenderWhere(dto({ region: 'DE2' }));
+
+    expect(where.AND).toEqual(
+      expect.arrayContaining([{ region: { startsWith: 'DE2' } }]),
+    );
+  });
+
+  it('plz/bundesland/region are omitted from AND when unset', () => {
+    const where = buildTenderWhere(dto());
+
+    const and = (where.AND as unknown[]) ?? [];
+    const hasGeoClause = and.some(
+      (clause) =>
+        typeof clause === 'object' &&
+        clause !== null &&
+        ('plz' in (clause as Record<string, unknown>) ||
+          'bundesland' in (clause as Record<string, unknown>) ||
+          'region' in (clause as Record<string, unknown>)),
+    );
+    expect(hasGeoClause).toBe(false);
+  });
 });
 
 describe('buildOrderBy', () => {