feat: add AI executive summary and remediation endpoint

[SHAURYAKSHARMA24]

Summary

Implements #81 by adding a secure AI insights endpoint.

Changes

• Adds POST /api/ai/insights
• Accepts provider, api_key, and findings
• Returns executive_summary and remediation_plan
• Adds AI provider abstraction for Anthropic, Groq, and Gemini
• Sorts findings by severity before generating the remediation prompt
• Registers the AI blueprint in api/app.py
• Adds pytest unit tests with mocked provider responses

Security

• JWT auth is enforced through the existing global /api/... auth middleware
• User API keys are not stored
• User API keys are not logged
• User API keys are not returned in responses
• Provider failure logs use a fixed safe message and provider name only
• Provider failures return a safe 502 response

Tests

• python -m py_compile api/providers/ai_provider.py
• python -m py_compile api/routes/ai.py
• python -m pytest tests/test_ai_insights.py -v
• python -m pytest tests/ -v

Closes #81

[Vishnu2707]

One note: api_key is accepted in the request body over the wire but make sure the transport is always HTTPS in deployment docs. Nothing blocking merge.