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>

docker-compose.yml

@@ -81,6 +81,14 @@ services:
       - DEEPL_API_KEY=${DEEPL_API_KEY:-}
       - OPENAI_API_KEY=${OPENAI_API_KEY:-}
       - OPENROUTER_API_KEY=${OPENROUTER_API_KEY:-}
+      - DEEPSEEK_ENABLED=${DEEPSEEK_ENABLED:-false}
+      - DEEPSEEK_API_KEY=${DEEPSEEK_API_KEY:-}
+      - DEEPSEEK_MODEL=${DEEPSEEK_MODEL:-deepseek-chat}
+      - DEEPSEEK_BASE_URL=${DEEPSEEK_BASE_URL:-https://api.deepseek.com/v1}
+      - MINIMAX_ENABLED=${MINIMAX_ENABLED:-false}
+      - MINIMAX_API_KEY=${MINIMAX_API_KEY:-}
+      - MINIMAX_MODEL=${MINIMAX_MODEL:-MiniMax-M1}
+      - MINIMAX_BASE_URL=${MINIMAX_BASE_URL:-https://api.minimax.chat/v1}
       - MAX_FILE_SIZE_MB=${MAX_FILE_SIZE_MB:-50}
       - RATE_LIMIT_REQUESTS_PER_MINUTE=${RATE_LIMIT_REQUESTS_PER_MINUTE:-60}
       - RATE_LIMIT_TRANSLATIONS_PER_MINUTE=${RATE_LIMIT_TRANSLATIONS_PER_MINUTE:-10}