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/app/admin/ProviderBreakdownChart.tsx

@@ -17,14 +17,12 @@ interface ProviderBreakdownChartProps {
 
 const providerLabels: Record<string, string> = {
   google: "Google Translate",
-  deepl: "DeepL",
   ollama: "Ollama (Local)",
   openai: "OpenAI",
 };
 
 const providerColors: Record<string, string> = {
   google: "bg-blue-500",
-  deepl: "bg-indigo-500",
   ollama: "bg-green-500",
   openai: "bg-purple-500",
 };