Rewrite firmware-api-spec as unified cross-consumer contract

Expands the frontend-only spec into the canonical contract covering
all consumers: web flashtool, ESP32 hub OTA, backend API, and CLI
tooling. Adds OTA update endpoint, CI/CD ingestion with markdown
changelog parsing, structured release notes, per-endpoint caching
strategy, consumer integration guidance, and schema evolution rules.

Author: hhvrc