[grade=B] fix: i18n detectLanguage RFC 7231 q-value compliance + stale test fixes

- Fix detectLanguage() to sort by HTTP q-values per RFC 7231 (was first-match-wins)
- Strict qvalue grammar: /^(0(?:\.\d{0,3})?|1(?:\.0{0,3})?)$/
- Exclude q=0 entries (not acceptable per RFC)
- Case-insensitive Q parameter name
- Fix 5 stale tests: zh/ja now supported (31 languages, not 5)
- Add 7 boundary regression tests for q-value parsing
- All 1781 tests pass

Codex grade: B (urn:ump:6yumklcezgiaemcg5t2mebuoi4w2n5dexozm4j7h5pu5g2s4p5ta)

dashcaddy-api/__tests__/routes/i18n-routes.test.js

@@ -14,13 +14,13 @@ function createI18nApp() {
 }
 
 describe('DC-077: i18n Routes', () => {
-  it('GET /i18n/languages returns 5 languages', async () => {
+  it('GET /i18n/languages returns 31 languages', async () => {
     const app = createI18nApp();
     const res = await request(app).get('/api/v1/i18n/languages');
 
     expect(res.status).toBe(200);
     expect(res.body.success).toBe(true);
-    expect(res.body.languages).toHaveLength(5);
+    expect(res.body.languages).toHaveLength(31);
     expect(res.body.default).toBe('en');
   });