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/providerTheme.json

@@ -22,8 +22,6 @@
   "providerTheme.default.desc": "ترجمة بنموذج لغوي كبير (LLM) مع تحليل دلالي متقدم.",
   "providerTheme.classic.google.label": "ترجمة Google",
   "providerTheme.classic.google.desc": "ترجمة فائقة السرعة تغطي أكثر من 130 لغة. موصى بها للتدفقات العامة.",
-  "providerTheme.classic.deepl.label": "DeepL Pro",
-  "providerTheme.classic.deepl.desc": "ترجمة عالية الدقة مشهور بسيولتها وصياغاتها الطبيعية.",
   "providerTheme.classic.google_cloud.label": "Google Cloud API",
   "providerTheme.classic.google_cloud.desc": "محرك سحابي احترافي محسّن لمعالجة كميات كبيرة من المستندات."
 }