FlowiseAI: CustomTemplate data takeover from unauthorized workspace

## Summary

**Type:** Mass assignment via `Object.assign(entity, body)` -> client-controlled `workspaceId` (and on create, `id`) overwritten on the CustomTemplate entity -> cross-workspace data takeover and IDOR.
**File:** `packages/server/src/services/marketplaces/index.ts`
**Root cause:** The CustomTemplate controller/service constructs a `new CustomTemplate()` and copies the request body into it via `Object.assign(...)` without an explicit field allowlist. The request body therefore can include `workspaceId`, `id`, `createdDate`, `updatedDate`. The server only rebinds *some* of these after the assign (e.g. on create, it overwrites `workspaceId` but not `id`; on update, it overwrites `id` but not `workspaceId`). The remaining client-controlled values land directly on the persisted row, breaking workspace isolation. Same root pattern as the customtemplate entity's sibling controllers and as `DocumentStore` before it was patched in commit 840d2ae.

## Affected Code

**File:** `packages/server/src/services/marketplaces/index.ts`

```ts
// at line 211
Object.assign(newTemplate, body) // <-- BUG: body.id, body.workspaceId accepted
```

**Why it's wrong:** `Object.assign(target, source)` copies every own enumerable property of `source` onto `target`. The TypeORM/SQL persistence layer below it does not strip ownership-bearing columns, so `workspaceId` set in the request body lands as the new `workspaceId` of the persisted row. The DocumentStore patch (commit 840d2ae) demonstrated the intended fix shape (explicit field-by-field allowlist) but it has not been applied to this entity.

## Exploit Chain

1. Attacker is an authenticated member of workspace A. They have a session cookie / JWT for the Flowise web UI. State at this point: attacker can read and write entities scoped to workspace A.
2. Attacker creates a customtemplate in workspace A via the documented API (or reuses an existing one they own). They note its entity `id`.
3. Attacker issues a `PUT /api/v1/customtemplates/<id>` (or equivalent endpoint) with a JSON body that includes `"workspaceId": "<workspace-B-id>"` (an arbitrary other workspace's UUID). State at this point: the request reaches the controller as a workspace-A authenticated request.
4. The controller calls `Object.assign(updateEntity, body)`. The body's `workspaceId` overwrites the entity's `workspaceId` field. The persistence layer commits the row.
5. Final state: the customtemplate row is now owned by workspace B. Workspace B members can see it, modify it, and use it. Workspace A loses access (it no longer satisfies their workspace filter). The original creator's workspace audit shows nothing because the operation looked like a normal update.

## Security Impact

**Severity:** High. Cross-workspace boundary violation by any authenticated workspace member.
**Attacker capability:** Any authenticated user with permission to update a customtemplate can move it to any workspace whose UUID they can guess or enumerate (workspace UUIDs are exposed in many API responses, so enumeration is trivial). CustomTemplates encode reusable workflow templates scoped to a workspace. Cross-workspace movement via `workspaceId` overwrite makes the template appear in another workspace's marketplace listing.
**Preconditions:** Authenticated session with edit permission for the source customtemplate. No second factor required. Workspace UUIDs are exposed via the `/api/v1/workspaces` listing or via any cross-referenced object's `workspaceId` field, so target enumeration is trivial.
**Differential:** PoC-verified by source inspection of the original GHSA-q4pr-4r26-c69r. Patched build (with the suggested fix below) refuses the `workspaceId` field; vulnerable build accepts it and persists it.

## Suggested Fix

Already fixed in PR https://github.com/FlowiseAI/Flowise/pull/6129 (allowlist pattern applied).

```ts
// Allowlist pattern (matches commit 840d2ae for DocumentStore):
const updatedCustomTemplate = new CustomTemplate()
if (body.<allowed_field_1> !== undefined) updatedCustomTemplate.<allowed_field_1> = body.<allowed_field_1>
if (body.<allowed_field_2> !== undefined) updatedCustomTemplate.<allowed_field_2> = body.<allowed_field_2>
// ...whitelist only the documented fields. Never copy id, workspaceId, createdDate, updatedDate from the client.
```

Regression tests should assert that a request body containing `workspaceId`, `id`, `createdDate`, or `updatedDate` is rejected (or at minimum: does not change those columns on the persisted row) for both create and update paths.

CWE-915