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.

README.md

@@ -0,0 +1,13 @@
+# logging
+
+Shared logging primitives for Shiny backend services.
+
+- `SetupLogger(level, format, service, version)` — configures the slog default
+  logger (text / json / otel) and returns it.
+- `ContextWithLogger` / `LoggerFromContext` — carry a logger on the context.
+- `NewMockLogger()` — test helper for asserting log output.
+- `logging/middleware.RequestLogger(logger)` — HTTP middleware that debug-logs
+  request/response bodies.
+
+Replaces the `logging` package (and the `middleware` request-logger) copied
+into the backend services.