Feature: Settings page for API keys and system configuration

[nullEFFORT]

Description

Add a Settings page to the dashboard for managing:

• LLM API keys (OpenAI, Anthropic)
• LLM provider selection
• Risk scoring thresholds
• NCMEC reporting credentials
• Enrichment toggles (OSINT, auto-enrichment)
• School hours configuration

Current State

API keys are managed via environment variables / .env file. This works but requires server restart for changes.

Proposed

• New /settings route in dashboard
• Backend API endpoint for reading/updating settings
• Sensitive fields (API keys) shown as masked with reveal toggle
• Changes take effect without restart where possible

Priority

Low — env vars work fine for now. This is a quality-of-life improvement.