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/middleware.go

@@ -212,15 +212,15 @@ func Auth(jwtSecret string) gin.HandlerFunc {
 		}
 
 		if tokenErr != "" {
-			c.JSON(http.StatusUnauthorized, gin.H{"error": tokenErr})
+			c.JSON(http.StatusUnauthorized, gin.H{"error": tokenErr, "code": "INVALID_TOKEN"})
 			c.Abort()
 			return
 		}
 
 		if hasToken {
-			c.JSON(http.StatusUnauthorized, gin.H{"error": "Invalid token"})
+			c.JSON(http.StatusUnauthorized, gin.H{"error": "Invalid token", "code": "INVALID_TOKEN"})
 		} else {
-			c.JSON(http.StatusUnauthorized, gin.H{"error": "Authentication required"})
+			c.JSON(http.StatusUnauthorized, gin.H{"error": "Authentication required", "code": "UNAUTHENTICATED"})
 		}
 		c.Abort()
 	}
@@ -245,12 +245,12 @@ func OptionalAuth(jwtSecret string) gin.HandlerFunc {
 func RequireAdmin() gin.HandlerFunc {
 	return func(c *gin.Context) {
 		if _, exists := c.Get("user_id"); !exists {
-			c.JSON(http.StatusUnauthorized, gin.H{"error": "Authentication required"})
+			c.JSON(http.StatusUnauthorized, gin.H{"error": "Authentication required", "code": "UNAUTHENTICATED"})
 			c.Abort()
 			return
 		}
 		if !c.GetBool("is_admin") {
-			c.JSON(http.StatusForbidden, gin.H{"error": "Admin access required"})
+			c.JSON(http.StatusForbidden, gin.H{"error": "Admin access required", "code": "ADMIN_REQUIRED"})
 			c.Abort()
 			return
 		}