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/projects.go

@@ -8,7 +8,6 @@ import (
 	"database/sql"
 	"errors"
 	"net/http"
-	"strconv"
 	"strings"
 	"time"
 
@@ -55,18 +54,9 @@ func handleGetProjects(c *gin.Context) {
 	db := c.MustGet("db").(*database.DB)
 	queries := sqlcdb.New(db.DB)
 
-	page, _ := strconv.Atoi(c.DefaultQuery("page", "1"))
-	limit, _ := strconv.Atoi(c.DefaultQuery("limit", "10"))
+	offset, limit := pageWindow(c, 10, 100)
 	search := strings.TrimSpace(c.DefaultQuery("search", ""))
 
-	if page < 1 {
-		page = 1
-	}
-	if limit > 100 || limit < 1 {
-		limit = 10
-	}
-	offset := (page - 1) * limit
-
 	searchParam := sql.NullString{}
 	if search != "" {
 		searchParam = sql.NullString{String: search, Valid: true}
@@ -99,15 +89,9 @@ func handleGetProjects(c *gin.Context) {
 		return
 	}
 
-	totalCount := int(total)
 	c.JSON(http.StatusOK, gin.H{
-		"projects": projects,
-		"pagination": gin.H{
-			"page":  page,
-			"limit": limit,
-			"total": totalCount,
-			"pages": (totalCount + limit - 1) / limit,
-		},
+		"projects":   projects,
+		"pagination": paginationMeta(offset, limit, int(total)),
 	})
 }