fix: system env vars silently overriding .env — load_dotenv(override=True)

Root cause: load_dotenv() default override=False meant system-level
ANTHROPIC_BASE_URL (https://api.deepseek.com/anthropic) took precedence
over .env's OPENAI_BASE_URL (https://api.minimaxi.com/anthropic). All
Anthropic API calls went to DeepSeek with a MiniMax key, causing 401.

Changes:
- backend/llm.py: load_dotenv(override=True) — .env always wins
- .env.example: add explicit ANTHROPIC_API_KEY + ANTHROPIC_BASE_URL
- CLAUDE.md: document env var priority pitfall

.env.example

@@ -2,12 +2,17 @@
 LLM_BACKEND=cloud
 
 # 云端提供商：openai 或 anthropic
-LLM_PROVIDER=openai
+LLM_PROVIDER=anthropic
 
-# 云端配置（OpenAI 兼容）
+# Anthropic 兼容 API（MiniMax 等，优先使用）
+ANTHROPIC_API_KEY=sk-xxxx
+ANTHROPIC_BASE_URL=https://api.minimaxi.com/anthropic
+
+# OpenAI 兼容 API（fallback，当 ANTHROPIC_* 未设置时使用）
 OPENAI_API_KEY=sk-xxxx
 OPENAI_BASE_URL=https://api.openai.com/v1
-LLM_MODEL=gpt-4o
+
+LLM_MODEL=MiniMax-M2.7
 
 # 本地大语言模型（Ollama）
 LOCAL_LLM_MODEL=qwen2.5-coder:7b