feat: add Prometheus metrics for Store queries

Add InstrumentedStore decorator that wraps any Store and records
per-method query duration histograms and error counters. Wired into
main.go so all storage consumers get automatic observability.

Bump version to 0.18.0.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

go.mod

@@ -9,6 +9,7 @@ require (
 	github.com/google/uuid v1.6.0
 	github.com/oschwald/maxminddb-golang v1.13.1
 	github.com/prometheus/client_golang v1.23.2
+	github.com/prometheus/client_model v0.6.2
 	golang.org/x/crypto v0.48.0
 	modernc.org/sqlite v1.45.0
 )
@@ -33,7 +34,6 @@ require (
 	github.com/muesli/termenv v0.16.0 // indirect
 	github.com/munnerz/goautoneg v0.0.0-20191010083416-a7dc8b61c822 // indirect
 	github.com/ncruces/go-strftime v1.0.0 // indirect
-	github.com/prometheus/client_model v0.6.2 // indirect
 	github.com/prometheus/common v0.66.1 // indirect
 	github.com/prometheus/procfs v0.16.1 // indirect
 	github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec // indirect