feat(api): error codes, idempotency keys, cursor pagination

- Stable machine-readable error codes alongside the error message
  (UNAUTHENTICATED, INVALID_TOKEN, ADMIN_REQUIRED, SCOPE_INSUFFICIENT,
  IDEMPOTENCY_IN_PROGRESS); CLI renders "CODE: message".
- Idempotency middleware on authed/admin groups: POST requests with an
  Idempotency-Key header replay the first response for 24h (Redis-backed,
  caller-scoped); concurrent duplicates get 409. X-Idempotent-Replay
  header marks replays.
- Shared pagination helpers: ?cursor= (opaque, preferred) alongside
  legacy ?page= on projects and deployments lists; pagination meta
  includes next_cursor; invalid cursors fail soft to page 1.
- CLI --idempotency-key flag for safe scripted retries.
- OpenAPI updated: error envelope, cursor params, idempotency contract.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

app/backend/internal/middleware/idempotency_test.go

@@ -0,0 +1,60 @@
+package middleware
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/gin-gonic/gin"
+)
+
+func TestIdempotencyPassThroughWithoutKey(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	called := 0
+	router := gin.New()
+	router.Use(Idempotency())
+	router.POST("/x", func(c *gin.Context) { called++; c.JSON(200, gin.H{"ok": true}) })
+
+	req := httptest.NewRequest(http.MethodPost, "/x", nil)
+	rec := httptest.NewRecorder()
+	router.ServeHTTP(rec, req)
+
+	if called != 1 || rec.Code != 200 {
+		t.Fatalf("expected pass-through, called=%d code=%d", called, rec.Code)
+	}
+}
+
+func TestIdempotencyPassThroughWithoutRedis(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	called := 0
+	router := gin.New()
+	router.Use(Idempotency())
+	router.POST("/x", func(c *gin.Context) { called++; c.JSON(200, gin.H{"ok": true}) })
+
+	// Key present but no redis in context — must not block the request.
+	req := httptest.NewRequest(http.MethodPost, "/x", nil)
+	req.Header.Set("Idempotency-Key", "abc-123")
+	rec := httptest.NewRecorder()
+	router.ServeHTTP(rec, req)
+
+	if called != 1 || rec.Code != 200 {
+		t.Fatalf("expected pass-through without redis, called=%d code=%d", called, rec.Code)
+	}
+}
+
+func TestIdempotencySkipsNonPOST(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	called := 0
+	router := gin.New()
+	router.Use(Idempotency())
+	router.GET("/x", func(c *gin.Context) { called++; c.String(200, "ok") })
+
+	req := httptest.NewRequest(http.MethodGet, "/x", nil)
+	req.Header.Set("Idempotency-Key", "abc")
+	rec := httptest.NewRecorder()
+	router.ServeHTTP(rec, req)
+
+	if called != 1 {
+		t.Fatalf("GET should bypass idempotency, called=%d", called)
+	}
+}