feat(applog): add JSON logger and context helpers

backend/internal/applog/applog.go

@@ -0,0 +1,51 @@
+// Package applog provides geniusrun's structured JSON logging: a
+// log/slog-based logger writing to stdout, plus context helpers so a
+// logger enriched in one layer (e.g. internal/api's HTTP middleware,
+// attaching a request_id) is picked up by another (e.g. internal/garmin's
+// wrapper-call logging) without either package depending on the other.
+package applog
+
+import (
+	"context"
+	"io"
+	"log/slog"
+	"strings"
+)
+
+// NewLogger builds a JSON-handler *slog.Logger writing to w at the given
+// level ("debug"|"info"|"warn"|"error", case-insensitive; anything else
+// defaults to info).
+func NewLogger(level string, w io.Writer) *slog.Logger {
+	return slog.New(slog.NewJSONHandler(w, &slog.HandlerOptions{Level: parseLevel(level)}))
+}
+
+func parseLevel(level string) slog.Level {
+	switch strings.ToLower(level) {
+	case "debug":
+		return slog.LevelDebug
+	case "warn":
+		return slog.LevelWarn
+	case "error":
+		return slog.LevelError
+	default:
+		return slog.LevelInfo
+	}
+}
+
+type ctxKey struct{}
+
+// WithLogger returns a context carrying logger, retrievable via FromContext.
+func WithLogger(ctx context.Context, logger *slog.Logger) context.Context {
+	return context.WithValue(ctx, ctxKey{}, logger)
+}
+
+// FromContext returns the logger stashed by WithLogger, or slog.Default()
+// if none was -- callers (e.g. the background incremental-sync loop, or
+// tests that don't bother injecting one) always get a working logger,
+// never nil.
+func FromContext(ctx context.Context) *slog.Logger {
+	if logger, ok := ctx.Value(ctxKey{}).(*slog.Logger); ok {
+		return logger
+	}
+	return slog.Default()
+}