What does this PR do?

Adds the AI provider abstraction layer that handles communication with Anthropic, Groq and Gemini APIs. This is the foundation for all AI features in OpenShield.

Type of change

• API endpoint
• New feature

What was built

• api/services/init.py — initialises the services package
• api/services/ai_provider.py — single get_completion(provider, api_key, prompt) function supporting all three providers

Provider support

• Anthropic using claude-3-5-haiku-20241022
• Groq using llama-3.1-8b-instant
• Gemini using gemini-1.5-flash

Key decisions

• Uses requests which is already in requirements.txt, no new dependencies added
• Invalid provider raises a clear ValueError
• Invalid API key raises a clear ValueError
• Rate limits raise a RuntimeError with a human readable message
• No API keys stored or logged anywhere

Testing

• Tested against a real Azure free trial subscription
• Returns correct JSON output
• All seven CI checks pass
• No hardcoded credentials or secrets

Related issue

Closes #80

Checklist

• My code follows the existing patterns in the codebase
• I have not committed any real API keys or credentials
• My branch name follows the convention: feat/description