feat(observability): configurable logging, request ids and error logs

- internal/logging: LOG_LEVEL/LOG_FORMAT parsing and slog handler
- request_id middleware (X-Request-ID or generated, echoed) + debug access log
- httpx registers errors; middleware logs internal causes with request_id
- health excluded, objects/sse debug-only, no query/body/secrets in logs
- new observability capability spec; README/.env.example docs

backend/internal/httpx/httpx.go

@@ -53,6 +53,7 @@ func Internal(message string) *AppError {
 }
 
 func Abort(c *gin.Context, err error) {
+	_ = c.Error(err)
 	var appErr *AppError
 	if errors.As(err, &appErr) {
 		c.AbortWithStatusJSON(appErr.Status, ErrorResponse{ErrorBody{appErr.Code, appErr.Message}})
@@ -62,6 +63,7 @@ func Abort(c *gin.Context, err error) {
 }
 
 func RespondError(c *gin.Context, err error) {
+	_ = c.Error(err)
 	var appErr *AppError
 	if errors.As(err, &appErr) {
 		c.JSON(appErr.Status, ErrorResponse{ErrorBody{appErr.Code, appErr.Message}})

backend/internal/httpx/httpx_test.go

@@ -0,0 +1,68 @@
+package httpx
+
+import (
+	"errors"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/gin-gonic/gin"
+)
+
+func runHandler(t *testing.T, handler gin.HandlerFunc) (*gin.Context, *httptest.ResponseRecorder) {
+	t.Helper()
+	gin.SetMode(gin.TestMode)
+	w := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(w)
+	c.Request = httptest.NewRequest(http.MethodGet, "/", nil)
+	handler(c)
+	return c, w
+}
+
+func TestRespondErrorRegistersInternalError(t *testing.T) {
+	c, w := runHandler(t, func(c *gin.Context) {
+		RespondError(c, errors.New("boom"))
+	})
+	if len(c.Errors) != 1 {
+		t.Fatalf("c.Errors = %d, want 1", len(c.Errors))
+	}
+	if w.Code != http.StatusInternalServerError {
+		t.Fatalf("status = %d, want 500", w.Code)
+	}
+	if body := w.Body.String(); body != `{"error":{"code":"internal_error","message":"internal server error"}}` {
+		t.Fatalf("body = %s", body)
+	}
+}
+
+func TestRespondErrorRegistersAppError(t *testing.T) {
+	c, w := runHandler(t, func(c *gin.Context) {
+		RespondError(c, NotFound("photo_not_found", "photo not found"))
+	})
+	if len(c.Errors) != 1 {
+		t.Fatalf("c.Errors = %d, want 1", len(c.Errors))
+	}
+	if w.Code != http.StatusNotFound {
+		t.Fatalf("status = %d, want 404", w.Code)
+	}
+}
+
+func TestAbortRegistersInternalError(t *testing.T) {
+	c, w := runHandler(t, func(c *gin.Context) {
+		Abort(c, errors.New("kaboom"))
+	})
+	if len(c.Errors) != 1 {
+		t.Fatalf("c.Errors = %d, want 1", len(c.Errors))
+	}
+	if w.Code != http.StatusInternalServerError {
+		t.Fatalf("status = %d, want 500", w.Code)
+	}
+}
+
+// Ensure the package-level recovery of AppError still works for logging levels.
+func TestInternalIsAppError500(t *testing.T) {
+	err := Internal("oops")
+	var appErr *AppError
+	if !errors.As(err, &appErr) || appErr.Status != http.StatusInternalServerError {
+		t.Fatalf("Internal() = %#v", err)
+	}
+}