Add generic /api/mask endpoint for standalone text masking

Add generic /api/mask endpoint for standalone text masking

Adds a new POST /api/mask endpoint that can be used by browser extensions,
CLIs, or any client that needs to mask PII and secrets independently of
the OpenAI/Anthropic proxy routes.

Features:
- Detects and masks both PII and secrets (configurable via detect param)
- Returns context mapping for client-side unmasking
- Supports multi-turn conversations via startFrom counters
- Auto-detects language or accepts explicit language parameter
- Logs requests to dashboard for visibility
- Rejects whitespace-only text input
- Consistent error handling for both PII and secrets detection