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/errors.go

@@ -0,0 +1,29 @@
+package api
+
+import (
+	"github.com/gin-gonic/gin"
+)
+
+// Stable machine-readable error codes for agent clients. The "error" field
+// stays human-readable; "code" is the contract the CLI/MCP rely on.
+const (
+	CodeUnauthorized       = "UNAUTHENTICATED"
+	CodeInvalidToken       = "INVALID_TOKEN"
+	CodeForbidden          = "FORBIDDEN"
+	CodeAdminRequired      = "ADMIN_REQUIRED"
+	CodeScopeInsufficient  = "SCOPE_INSUFFICIENT"
+	CodeNotFound           = "NOT_FOUND"
+	CodeValidation         = "VALIDATION"
+	CodeConflict           = "CONFLICT"
+	CodeInternal           = "INTERNAL"
+	CodeUnavailable        = "DEPENDENCY_UNAVAILABLE"
+	CodeIdempotencyRunning = "IDEMPOTENCY_IN_PROGRESS"
+)
+
+// respondError emits the standard error envelope. Existing handlers return
+// bare {"error": msg} — this adds the stable code for new and
+// agent-facing paths.
+func respondError(c *gin.Context, status int, code, message string) {
+	c.JSON(status, gin.H{"error": message, "code": code})
+	c.Abort()
+}