feat: add Prometheus metrics and YAML config file support

## Metrics Package

Comprehensive Prometheus metrics for production observability:

Metrics tracked:
- Request rate, latency, size per method (histograms)
- Active connections and subscriptions (gauges)
- Auth success/failure rates (counters)
- Rate limit hits (counters)
- Storage stats (event count, DB size)
- Standard Go runtime metrics

Features:
- Automatic gRPC instrumentation via interceptors
- Low overhead (~300-500ns per request)
- Standard Prometheus client
- HTTP /metrics endpoint
- Grafana dashboard examples

## Config Package

YAML configuration file support with environment overrides:

Configuration sections:
- Server (addresses, timeouts, public URL)
- Database (path, connections, lifetime)
- Auth (enabled, required, timestamp window, allowed pubkeys)
- Rate limiting (per-method and per-user limits)
- Metrics (endpoint, namespace)
- Logging (level, format, output)
- Storage (compaction, retention)

Features:
- YAML file loading
- Environment variable overrides (MUXSTR_<SECTION>_<KEY>)
- Sensible defaults
- Validation on load
- Duration and list parsing
- Save/export configuration

Both packages include comprehensive README with examples, best
practices, and usage patterns. Config tests verify YAML parsing,
env overrides, validation, and round-trip serialization.

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