Add telemetry ingestion backend and production deployment

[Icemap]

Summary

Add the product-owned telemetry ingestion backend, including strict event validation, bounded in-memory batching, TiDB storage, personless PostHog forwarding, health and private metrics endpoints, and Docker Compose/Caddy deployment.

Add CI coverage for the backend binary and a manually triggered production deployment workflow using the telemetry-production GitHub Environment. Runtime application credentials remain only in the server-local .env; GitHub stores only SSH deployment credentials.

Update the telemetry specification, backend deployment design, README, and agent guidance to match the implementation. The default sink timeout is 5 seconds based on observed PostHog ingestion latency during pre-deployment validation.

Validation

• go test -count=1 ./...
• go test -count=1 -race ./internal/telemetrybackend
• go vet ./...
• Docker Compose configuration validation
• ARM64 Docker image build
• Live TiDB schema creation, insert, read, and cleanup
• Live HTTP ingestion returning 202 Accepted
• Live batch flush with successful TiDB and PostHog sink writes