fix: resolve all linting and formatting issues

- Fix error return value checks (errcheck)
- Fix unused parameters by using underscore prefix
- Add missing package comments to all packages
- Fix context key type issue in middleware (use typed contextKey)
- Replace deprecated trace.NewNoopTracerProvider with noop.NewTracerProvider
- Fix embedded field selector in database client
- Remove trailing whitespace
- Remove revive linter (as requested) to avoid stuttering warnings for public API interfaces

All linting and formatting checks now pass.

internal/metrics/metrics.go

@@ -1,3 +1,4 @@
+// Package metrics provides Prometheus metrics collection and instrumentation.
 package metrics
 
 import (
@@ -12,9 +13,9 @@ import (
 // Metrics holds all Prometheus metrics.
 type Metrics struct {
 	httpRequestDuration *prometheus.HistogramVec
-	httpRequestTotal   *prometheus.CounterVec
-	httpErrorsTotal    *prometheus.CounterVec
-	registry           *prometheus.Registry
+	httpRequestTotal    *prometheus.CounterVec
+	httpErrorsTotal     *prometheus.CounterVec
+	registry            *prometheus.Registry
 }
 
 // NewMetrics creates a new metrics registry with all metrics.
@@ -94,4 +95,3 @@ func (m *Metrics) Handler() http.Handler {
 func (m *Metrics) Registry() *prometheus.Registry {
 	return m.registry
 }
-