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/observability/tracer.go

@@ -1,3 +1,4 @@
+// Package observability provides OpenTelemetry tracing setup and configuration.
 package observability
 
 import (
@@ -13,22 +14,23 @@ import (
 	sdktrace "go.opentelemetry.io/otel/sdk/trace"
 	semconv "go.opentelemetry.io/otel/semconv/v1.26.0"
 	"go.opentelemetry.io/otel/trace"
+	"go.opentelemetry.io/otel/trace/noop"
 )
 
 // Config holds OpenTelemetry configuration.
 type Config struct {
-	Enabled      bool
-	ServiceName  string
+	Enabled        bool
+	ServiceName    string
 	ServiceVersion string
-	Environment  string
-	OTLPEndpoint string
+	Environment    string
+	OTLPEndpoint   string
 }
 
 // InitTracer initializes OpenTelemetry tracing.
 func InitTracer(ctx context.Context, cfg Config) (trace.TracerProvider, error) {
 	if !cfg.Enabled {
 		// Return a no-op tracer provider
-		return trace.NewNoopTracerProvider(), nil
+		return noop.NewTracerProvider(), nil
 	}
 
 	// Create resource with service information
@@ -91,4 +93,3 @@ func ShutdownTracer(ctx context.Context, tp trace.TracerProvider) error {
 	}
 	return nil
 }
-