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/it/pricing.json

@@ -27,14 +27,14 @@
   "pricing.plans.free.feat5": "Supporto della community",
   "pricing.plans.starter.feat1": "50 documenti / mese",
   "pricing.plans.starter.feat2": "Fino a 50 pagine per documento",
-  "pricing.plans.starter.feat3": "Google Traduttore + DeepL",
+  "pricing.plans.starter.feat3": "Google Traduttore",
   "pricing.plans.starter.feat4": "File fino a 10 MB",
   "pricing.plans.starter.feat5": "Supporto via e-mail",
   "pricing.plans.starter.feat6": "Cronologia di 30 giorni",
   "pricing.plans.pro.feat1": "200 documenti / mese",
   "pricing.plans.pro.feat2": "Fino a 200 pagine per documento",
   "pricing.plans.pro.feat3": "Traduzione IA Essenziale",
-  "pricing.plans.pro.feat4": "Google Traduttore + DeepL",
+  "pricing.plans.pro.feat4": "Google Traduttore",
   "pricing.plans.pro.feat5": "File fino a 25 MB",
   "pricing.plans.pro.feat6": "Glossari personalizzati",
   "pricing.plans.pro.feat7": "Supporto prioritario",
@@ -83,7 +83,6 @@
   "pricing.comparison.pagesMaxPerDoc": "Pagine max / documento",
   "pricing.comparison.maxFileSize": "Dimensione max file",
   "pricing.comparison.googleTranslation": "Google Traduttore",
-  "pricing.comparison.deepl": "DeepL",
   "pricing.comparison.aiEssential": "Traduzione IA Essenziale",
   "pricing.comparison.aiPremium": "Traduzione IA Premium",
   "pricing.comparison.apiAccess": "Accesso API",