Make repeated channel-monitor setup safer

Admins often recreate monitors with the same endpoint, model, and request settings. A server-side duplicate keeps the stored API key out of the browser, creates a disabled copy for review, and uses stable operation identity to recover ambiguous retries without creating extra rows.

Constraint: Stored monitor API keys must never be returned to the browser
Constraint: Applying a request template must preserve internal duplicate recovery metadata
Rejected: Rebuild the monitor from list data | list responses only contain a masked API key
Rejected: Copy runtime state and history | a duplicate should start as an unverified configuration
Confidence: high
Scope-risk: moderate
Reversibility: clean
Directive: Keep duplicated monitors disabled until an administrator reviews and enables them
Tested: Go unit tests for repository, service, and admin handler; integration-tag compile; go vet; golangci-lint v2.9; frontend Vitest, ESLint, typecheck, production build; Playwright duplicate flow
Not-tested: PostgreSQL container integration locally because Docker is unavailable; CI will execute the database-backed suite

backend/internal/handler/admin/channel_monitor_handler.go

@@ -1,6 +1,8 @@
 package admin
 
 import (
+	"context"
+	"log/slog"
 	"strconv"
 	"strings"
 	"time"
@@ -333,6 +335,63 @@ func (h *ChannelMonitorHandler) Create(c *gin.Context) {
 	response.Created(c, channelMonitorToResponse(m))
 }
 
+// Duplicate POST /api/v1/admin/channel-monitors/:id/duplicate
+func (h *ChannelMonitorHandler) Duplicate(c *gin.Context) {
+	id, ok := ParseChannelMonitorID(c)
+	if !ok {
+		return
+	}
+	subject, _ := middleware2.GetAuthSubjectFromContext(c)
+	actorScope := adminActorScope(c)
+
+	result, err := executeAdminIdempotent(
+		c,
+		"admin.channel_monitors.duplicate",
+		struct {
+			MonitorID int64 `json:"monitor_id"`
+		}{MonitorID: id},
+		service.DefaultWriteIdempotencyTTL(),
+		func(ctx context.Context) (any, error) {
+			monitor, err := h.monitorService.Duplicate(
+				ctx,
+				id,
+				subject.UserID,
+				actorScope,
+				c.GetHeader("Idempotency-Key"),
+			)
+			if err != nil {
+				return nil, err
+			}
+			return channelMonitorToResponse(monitor), nil
+		},
+	)
+	if err != nil {
+		reason := infraerrors.Reason(err)
+		if reason == infraerrors.Reason(service.ErrIdempotencyInProgress) || reason == infraerrors.Reason(service.ErrIdempotencyStoreUnavail) {
+			recovered, recoverErr := h.monitorService.RecoverDuplicate(
+				c.Request.Context(),
+				id,
+				actorScope,
+				c.GetHeader("Idempotency-Key"),
+			)
+			if recoverErr != nil {
+				slog.Warn("channel_monitor_duplicate_recovery_failed", "monitor_id", id, "actor_scope", actorScope, "reason", reason, "error", recoverErr)
+			} else if recovered != nil {
+				c.Header("X-Idempotency-Recovered", "true")
+				response.Success(c, channelMonitorToResponse(recovered))
+				return
+			}
+		}
+		response.ErrorFrom(c, err)
+		return
+	}
+
+	if result != nil && result.Replayed {
+		c.Header("X-Idempotency-Replayed", "true")
+	}
+	response.Success(c, result.Data)
+}
+
 // Update PUT /api/v1/admin/channel-monitors/:id
 func (h *ChannelMonitorHandler) Update(c *gin.Context) {
 	id, ok := ParseChannelMonitorID(c)