docs(api): add usage metering API reference and admin guide (#4488)

Comprehensive reference for the usage metering system introduced in #1807.
Covers all six endpoints, LLMUsageRecord data model, cost calculation
algorithm, MODEL_PRICING_PER_1M_TOKENS table, Redis key patterns and TTLs,
UsageView dashboard walkthrough, and admin setup/troubleshooting guide.

Author: mrveiss