feat(accounts): make repeated static account setup safer

Admins often need another account with the same provider and routing configuration. Duplicate on the server so credentials never return to the browser, preserve exact group priorities atomically, start the copy paused, and recover the same copy after ambiguous idempotency-store failures.

Constraint: Admin account responses redact credentials, so duplication must remain server-side

Constraint: OAuth and setup-token credentials rotate and must not be shared across account rows

Rejected: Copy raw account JSON to the clipboard | exposes credentials outside the server

Rejected: Duplicate rotating credentials | account-scoped refresh locks can race token rotation

Confidence: high

Scope-risk: moderate

Reversibility: clean

Directive: Keep copies paused, avoid automatic upstream probes, and exclude rotating credential types unless token ownership is redesigned

Tested: Targeted Go tests, Go vet, server build; frontend lint, typecheck, Vitest suite, production build; integration test compiled

Not-tested: Docker-backed PostgreSQL execution because Docker is unavailable

Related: Wei-Shaw/sub2api#1379

Related: Wei-Shaw/sub2api#2928

backend/internal/handler/admin/account_handler.go

@@ -860,6 +860,41 @@ func (h *AccountHandler) Create(c *gin.Context) {
 	response.Success(c, result.Data)
 }
 
+// Duplicate handles creating an independent account from an existing account's configuration.
+// POST /api/v1/admin/accounts/:id/duplicate
+func (h *AccountHandler) Duplicate(c *gin.Context) {
+	accountID, err := strconv.ParseInt(c.Param("id"), 10, 64)
+	if err != nil {
+		response.BadRequest(c, "Invalid account ID")
+		return
+	}
+
+	result, err := executeAdminIdempotent(
+		c,
+		"admin.accounts.duplicate",
+		struct {
+			AccountID int64 `json:"account_id"`
+		}{AccountID: accountID},
+		service.DefaultWriteIdempotencyTTL(),
+		func(ctx context.Context) (any, error) {
+			account, execErr := h.adminService.DuplicateAccount(ctx, accountID, c.GetHeader("Idempotency-Key"))
+			if execErr != nil {
+				return nil, execErr
+			}
+			return h.buildAccountResponseWithRuntime(ctx, account), nil
+		},
+	)
+	if err != nil {
+		response.ErrorFrom(c, err)
+		return
+	}
+
+	if result != nil && result.Replayed {
+		c.Header("X-Idempotency-Replayed", "true")
+	}
+	response.Success(c, result.Data)
+}
+
 // Update handles updating an account
 // PUT /api/v1/admin/accounts/:id
 func (h *AccountHandler) Update(c *gin.Context) {