P5: WhatsApp Classifier — Add Metering + Usage Tracking

The WhatsApp classifier service already exists on Cloud Run. Add metering middleware so every WhatsApp classification query is tracked as a billable event. Integration: 1. Wrap each classification response in MeterableRequest from c8_platform/billing/metering.py 2. Track: customer_id (phone number → customer mapping), request_type=HTS_CLASSIFY, tokens used, model used 3. Monthly usage summary per phone number / location 4. Rate limiting: free tier = 25/day, paid = based on subscription tier 5. Upsell trigger: when free tier exhausted, send WhatsApp message with pricing link Revenue model: Stream #21 — $0.05/query or $99-499/month per location Output: Updated WhatsApp handler with metering integration Target: c8_platform/askrosetta_api/whatsapp_handler.py (or wherever the WhatsApp service lives)