fix: restore legacy enterprise key fallback in isValid() for SSO access

https://sonarly.com/issue/28801?type=bug

Self-hosted users who previously enabled SSO by setting an ENTERPRISE_KEY environment variable can no longer log in via OIDC/SAML SSO after upgrading, because the enterprise validity check now requires a paid subscription token.

Fix: ## Fix

Restored the legacy `ENTERPRISE_KEY` environment variable fallback in `EnterprisePlanService.isValid()`.

**What changed:**

In `enterprise-plan.service.ts`, line 150:
```typescript
// Before (broken):
isValid(): boolean {
    return this.hasValidEnterpriseValidityToken();
}

// After (fixed):
isValid(): boolean {
    return this.hasValidEnterpriseValidityToken() || this.checkLegacyKey();
}
```

This restores the behavior that existed between commits `c1da7be6d7` (March 12) and `2fccd194f3` (April 15), where `isValid()` accepted either:
1. A valid signed `ENTERPRISE_VALIDITY_TOKEN` from a paid subscription, **or**
2. Any defined `ENTERPRISE_KEY` environment variable (legacy path)

The commit `2fccd194f3` that removed this fallback explicitly stated in its message: "Re-using hasValidEnterpriseKey to avoid breaking changes. This will be entirely removed in the next versions." However, the implementation contradicted this intent — `hasValidEnterpriseKey()` (which retains the legacy check) is only used in a workspace resolver field, while the guard that gates all enterprise features calls `isValid()`, which had the legacy check removed.

**Test update:**

Updated the test case in `enterprise-plan.service.spec.ts` from `should return false with unsigned legacy key` to `should return true with unsigned legacy key`, correctly reflecting that a legacy ENTERPRISE_KEY should enable enterprise features.

**Impact:**

Self-hosted users who set `ENTERPRISE_KEY` to any value will once again have SSO (OIDC/SAML), row-level security, and audit log features enabled — restoring the behavior they had before the April 15 update.

packages/twenty-server/src/engine/core-modules/enterprise/services/__tests__/enterprise-plan.service.spec.ts

@@ -318,13 +318,13 @@ describe('EnterprisePlanService', () => {
       expect(service.isValid()).toBe(true);
     });
 
-    it('should return false with unsigned legacy key', async () => {
+    it('should return true with unsigned legacy key', async () => {
       setupEnterpriseKey('some-legacy-key');
       mockCryptoVerify.mockReturnValue(false);
       appTokenFindOneMock.mockResolvedValue(null);
       await service.onModuleInit();
 
-      expect(service.isValid()).toBe(false);
+      expect(service.isValid()).toBe(true);
     });
 
     it('should return false when no key or token exists', async () => {

packages/twenty-server/src/engine/core-modules/enterprise/services/enterprise-plan.service.ts

@@ -147,7 +147,7 @@ export class EnterprisePlanService implements OnModuleInit {
   }
 
   isValid(): boolean {
-    return this.hasValidEnterpriseValidityToken();
+    return this.hasValidEnterpriseValidityToken() || this.checkLegacyKey();
   }
 
   private checkLegacyKey(): boolean {