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/nl/landing.json

@@ -52,7 +52,7 @@
   "landing.layout.p5.title": "Kop- en voetteksten",
   "landing.layout.p5.desc": "Koppen, voetteksten en voetnoten worden nooit over het hoofd gezien.",
   "landing.layout.p6.title": "130+ talen",
-  "landing.layout.p6.desc": "Google Translate, DeepL en professionele AI-engines.",
+  "landing.layout.p6.desc": "Google Translate en professionele AI-engines.",
   "landing.formats.title": "Elk formaat,",
   "landing.formats.title2": "elk element",
   "landing.formats.subtitle": "Wij vertalen wat anderen missen. Uw bedrijf verdient onberispelijke documentatie.",
@@ -93,7 +93,7 @@
   "landing.pricing.starter.desc": "Voor particulieren en kleine projecten",
   "landing.pricing.starter.f1": "50 documenten / maand",
   "landing.pricing.starter.f2": "Tot 50 pagina's per doc",
-  "landing.pricing.starter.f3": "Google Translate + DeepL",
+  "landing.pricing.starter.f3": "Google Translate",
   "landing.pricing.starter.f4": "Bestanden tot 10 MB",
   "landing.pricing.starter.cta": "Aan de slag",
   "landing.pricing.pro.name": "Pro",
@@ -101,7 +101,7 @@
   "landing.pricing.pro.f1": "200 documenten / maand",
   "landing.pricing.pro.f2": "Tot 200 pagina's per doc",
   "landing.pricing.pro.f3": "IA-aangedreven vertaling",
-  "landing.pricing.pro.f4": "Google + DeepL inbegrepen",
+  "landing.pricing.pro.f4": "Google inbegrepen",
   "landing.pricing.pro.f5": "Eigen glossaria en prompts",
   "landing.pricing.pro.f6": "Prioriteitsondersteuning",
   "landing.pricing.pro.cta": "Probeer Pro",