feat: initial shared logging module

slog SetupLogger (text/json/otel), context logger helpers, MockLogger test
helper, and a request-logger HTTP middleware sub-package. Replaces the logging
package + middleware request-logger copied across the backend services.

go.mod

@@ -0,0 +1,22 @@
+module gitea.unbound.se/shiny/logging
+
+go 1.25.0
+
+require (
+	github.com/stretchr/testify v1.11.1
+	go.opentelemetry.io/contrib/bridges/otelslog v0.19.0
+)
+
+require (
+	github.com/cespare/xxhash/v2 v2.3.0 // indirect
+	github.com/davecgh/go-spew v1.1.1 // indirect
+	github.com/go-logr/logr v1.4.3 // indirect
+	github.com/go-logr/stdr v1.2.2 // indirect
+	github.com/pmezard/go-difflib v1.0.0 // indirect
+	go.opentelemetry.io/auto/sdk v1.2.1 // indirect
+	go.opentelemetry.io/otel v1.44.0 // indirect
+	go.opentelemetry.io/otel/log v0.20.0 // indirect
+	go.opentelemetry.io/otel/metric v1.44.0 // indirect
+	go.opentelemetry.io/otel/trace v1.44.0 // indirect
+	gopkg.in/yaml.v3 v3.0.1 // indirect
+)