P0: Gateway Service — Auth + Routing + Metering Middleware

Build the Gateway service (rosetta-gateway) as a FastAPI application that handles authentication, routing to Ship/Shop/Shield, and metering middleware. Responsibilities: 1. API Key auth — validate X-API-Key against api_keys table, enforce rate limits 2. Salesforce JWT Bearer auth — validate SF JWT tokens for Connected App integration 3. Route fan-out — proxy /ship/* to Ship URL, /shop/* to Shop URL, /shield/* to Shield URL 4. Metering middleware — wrap every request in MeterableRequest, log to c8_usage_records 5. Health check — aggregate health from all downstream services 6. Salesforce webhook — POST /webhooks/salesforce receives Pub/Sub push, publishes SF Platform Events 7. OData endpoints — /odata/classifications, /odata/screenings for Salesforce Connect External Objects Reference: - c8-infra/openapi/rosetta-api.yaml — full API spec - c8_platform/billing/metering.py — metering integration - c8_platform/billing/cost_calculator.py — cost tracking - c8_platform/askrosetta_api/stripe_billing.py — Stripe webhook handling - c8_platform/askrosetta_api/gcp_marketplace.py — GCP Marketplace entitlement handling Output: Complete gateway/ directory with main.py, auth/, routes/, middleware/, requirements-gateway.txt Target: c8-infra/gateway/