User-Configurable Ban Lists & Custom Rules

LLM Guard → BanSubstrings, BanTopics, BanCompetitors, Regex, BanCode.

- Super easy win for idpishield

Add to Config:
```
BanSubstrings   []string
BanTopics       []string
BanCompetitors  []string
CustomRegex     []string
```

- Load from JSON/YAML config file or env vars (like IDPISHIELD_BAN_TOPICS=OpenAI,Anthropic).