feat: add slog structured logging

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

internal/api/logging_middleware.go

@@ -0,0 +1,52 @@
+package api
+
+import (
+	"log/slog"
+	"net/http"
+	"time"
+
+	"github.com/gin-gonic/gin"
+)
+
+// requestLogger returns a Gin middleware that logs each request via slog.
+// Requests to /healthz are skipped to avoid health-check noise.
+func requestLogger(lg *slog.Logger) gin.HandlerFunc {
+	if lg == nil {
+		lg = slog.Default()
+	}
+	return func(c *gin.Context) {
+		if c.FullPath() == "/healthz" || c.Request.URL.Path == "/healthz" {
+			c.Next()
+			return
+		}
+
+		start := time.Now()
+		c.Next()
+
+		status := c.Writer.Status()
+		path := c.FullPath()
+		if path == "" {
+			path = c.Request.URL.Path
+		}
+
+		attrs := []any{
+			"method", c.Request.Method,
+			"path", path,
+			"status", status,
+			"latency_ms", time.Since(start).Milliseconds(),
+			"client_ip", c.ClientIP(),
+		}
+		if len(c.Errors) > 0 {
+			attrs = append(attrs, "error", c.Errors.String())
+		}
+
+		switch {
+		case status >= http.StatusInternalServerError:
+			lg.Error("http request completed", attrs...)
+		case status >= http.StatusBadRequest:
+			lg.Warn("http request completed", attrs...)
+		default:
+			lg.Info("http request completed", attrs...)
+		}
+	}
+}