Add Prometheus metrics endpoint and backend instrumentation

[sleroq]

Summary

• add an in-process Prometheus registry in lib/app/metrics.ts with counters, gauges, histograms, and text exposition
• expose GET /metrics in the web server and instrument HTTP request totals/duration using low-cardinality route templates
• instrument Telegram update/error paths, AI generation flows (chat/notes/memory/character), usage-window events, and rate-limit exceed callbacks

Validation

• deno check --allow-import main.ts
• deno lint
• AI_TOKEN=\"test\" deno test -A
• scripts/build.bash