feat: add metric for failed admin audit inserts

[sergey3bv]

Summary

• Adds keycast_admin_audit_write_failures_total to Prometheus metrics;
• Increments it in fail-open admin_audit_events write paths, preserving availability while improving degradation visability.

Avoid naming corporate partners, customers, or other sensitive external brands in this PR title or body. Use generic descriptors unless a maintainer explicitly approves the public reference.

Motivation

• A follow-up for chore(admin): add durable audit trail for registered_clients admin changes #178

Related Issue

• Closes Follow-up: add observability for failed admin audit event inserts #184

Testing

• cargo test --workspace --verbose
• cargo clippy --workspace --all-targets --all-features -- -D warnings -A deprecated
• cargo fmt --all -- --check
• Manual verification completed

Visuals

• UI/web change with screenshots/video attached
• No visual change
• Visuals and text avoid sensitive external brand or partner names unless explicitly approved