feat(pricing): remove DeepL from all user-facing surfaces

Pricing: 'deepl' removed from plan provider lists and the comparison
table row. Landing and pricing copy (13 locales) no longer mention
DeepL (65 values cleaned, 39 dead keys deleted: pricing.comparison.deepl,
providerTheme.classic.deepl.*).

Providers: available-provider responses are filtered so a DeepL entry
from the backend can never surface in the engine picker or the services
page; DeepL theme entry and static lib/api.ts list entry removed.

Admin: DeepL config card, type fields, defaults and fallback-chain
mentions removed; stats/chart/status maps and mock data cleaned.

Backend adapter untouched — DeepL is invisible app-wide via the UI
filter; removing the Python adapter itself is a separate step if wanted.

Verified: build exit 0, vitest 9/9, eslint clean on touched files,
zero 'deepl' occurrences outside the two intentional UI filters.

frontend/src/lib/i18n/messages/ar/landing.json

@@ -52,7 +52,7 @@
   "landing.layout.p5.title": "الرؤوس والتذييلات",
   "landing.layout.p5.desc": "الرؤوس والتذييلات والحواشي لا تُفوت أبدًا.",
   "landing.layout.p6.title": "130+ لغة",
-  "landing.layout.p6.desc": "Google Translate و DeepL ومحركات ذكاء اصطناعي احترافية.",
+  "landing.layout.p6.desc": "Google Translate ومحركات ذكاء اصطناعي احترافية.",
   "landing.formats.title": "كل تنسيق،",
   "landing.formats.title2": "كل عنصر",
   "landing.formats.subtitle": "نترجم ما يفوته الآخرون. عملك يستحق توثيقًا لا تشوبه شائبة.",
@@ -93,7 +93,7 @@
   "landing.pricing.starter.desc": "للأفراد والمشاريع الصغيرة",
   "landing.pricing.starter.f1": "50 مستند / شهر",
   "landing.pricing.starter.f2": "حتى 50 صفحة لكل مستند",
-  "landing.pricing.starter.f3": "Google Translate + DeepL",
+  "landing.pricing.starter.f3": "Google Translate",
   "landing.pricing.starter.f4": "ملفات حتى 10 ميجابايت",
   "landing.pricing.starter.cta": "ابدأ الآن",
   "landing.pricing.pro.name": "Pro",
@@ -101,7 +101,7 @@
   "landing.pricing.pro.f1": "200 مستند / شهر",
   "landing.pricing.pro.f2": "حتى 200 صفحة لكل مستند",
   "landing.pricing.pro.f3": "ترجمة بالذكاء الاصطناعي",
-  "landing.pricing.pro.f4": "Google + DeepL مشمولان",
+  "landing.pricing.pro.f4": "Google مشمولان",
   "landing.pricing.pro.f5": "قواميس وأوامر مخصصة",
   "landing.pricing.pro.f6": "دعم ذو أولوية",
   "landing.pricing.pro.cta": "جرّب Pro",