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/api/pagination_test.go

@@ -0,0 +1,55 @@
+package api
+
+import (
+	"net/http/httptest"
+	"testing"
+
+	"github.com/gin-gonic/gin"
+)
+
+func ctxWithQuery(q string) *gin.Context {
+	gin.SetMode(gin.TestMode)
+	c, _ := gin.CreateTestContext(httptest.NewRecorder())
+	c.Request = httptest.NewRequest("GET", "/x?"+q, nil)
+	return c
+}
+
+func TestPageWindowDefaults(t *testing.T) {
+	offset, limit := pageWindow(ctxWithQuery(""), 10, 100)
+	if offset != 0 || limit != 10 {
+		t.Fatalf("expected 0/10, got %d/%d", offset, limit)
+	}
+}
+
+func TestPageWindowLegacyPage(t *testing.T) {
+	offset, limit := pageWindow(ctxWithQuery("page=3&limit=20"), 10, 100)
+	if offset != 40 || limit != 20 {
+		t.Fatalf("expected 40/20, got %d/%d", offset, limit)
+	}
+}
+
+func TestPageWindowCursor(t *testing.T) {
+	// cursor for offset 50 produced by paginationMeta
+	meta := paginationMeta(0, 50, 200)
+	cursor := meta["next_cursor"].(string)
+	offset, limit := pageWindow(ctxWithQuery("cursor="+cursor+"&limit=50"), 10, 100)
+	if offset != 50 || limit != 50 {
+		t.Fatalf("expected 50/50, got %d/%d", offset, limit)
+	}
+}
+
+func TestPageWindowInvalidCursorFailsSoft(t *testing.T) {
+	offset, _ := pageWindow(ctxWithQuery("cursor=garbage"), 10, 100)
+	if offset != 0 {
+		t.Fatalf("expected offset 0 on invalid cursor, got %d", offset)
+	}
+}
+
+func TestPaginationMetaNextCursorOnlyWhenMore(t *testing.T) {
+	if _, ok := paginationMeta(0, 10, 10)["next_cursor"]; ok {
+		t.Fatal("no next page should omit next_cursor")
+	}
+	if _, ok := paginationMeta(0, 10, 25)["next_cursor"]; !ok {
+		t.Fatal("expected next_cursor when more pages exist")
+	}
+}