feat: add DeepSeek and Minimax (m2.7) translation providers

New providers:
- DeepSeek: direct API with deepseek-chat model, very cost-effective
- Minimax: MiniMax-M1 model via OpenAI-compatible API, supports m2.7

Changes:
- Full provider implementations with retry, health check, batch support
- Provider config with env vars (DEEPSEEK_*, MINIMAX_*)
- Auto-registration in provider registry
- Updated fallback chain to include new providers
- Updated setup-env.sh wizard with options 6 (deepseek) and 7 (minimax)
- Updated manage-keys.sh with new menu entries and provider switching
- Updated docker-compose.yml with new env vars

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

services/providers/__init__.py

@@ -69,6 +69,16 @@ def _auto_register_providers() -> None:
 
         register_openai_provider()
 
+    if ProvidersConfig.DEEPSEEK_ENABLED and ProvidersConfig.DEEPSEEK_API_KEY:
+        from .deepseek_provider import register_deepseek_provider
+
+        register_deepseek_provider()
+
+    if ProvidersConfig.MINIMAX_ENABLED and ProvidersConfig.MINIMAX_API_KEY:
+        from .minimax_provider import register_minimax_provider
+
+        register_minimax_provider()
+
 
 _auto_register_providers()