bug: OPA/RBAC authorization performance degrades significantly when user belongs to many organizations

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Current Behavior

Reported by customer on ticket 4943.

After upgrading from Coder 2.24.x to 2.28.8, a customer is experiencing significant UI latency (7+ seconds) when loading the Workspaces page. The issue is specific to users who belong to many organizations (108 in this case).

Key observations from the customer:

- Database migrations completed successfully (schema version 393)
- Database latency appears normal according to their monitoring
- The `/api/v2/organizations` endpoint takes ~3 seconds (verified via curl)
- CPU profiling (pprof) shows high CPU usage in OPA (Open Policy Agent) functions:
- ast.(*GenericVisitor).Walk - `12.53% flat, 52.88% cumulative`
- topdown.(*saveSet).containsrec.func1 - `2.27% flat, 23.37% cumulative`
- topdown.(*saveSet).saveRequired.func1 - `2.03% flat, 49.93% cumulative`
- Various ast.WalkTerms and evaluation functions
- CPU throttling was observed
- When logged in as a user assigned to only a single organization, there are no latency issues

We have been able to reproduce this issue in house:

1. Create a Coder deployment
2. Create 100+ organizations
3. Add a user to all organizations
4. Log in as that user
5. Navigate to the Workspaces page (the user should have atleast 3-4 workspaces although the latency is noticeable with just 1 workspace as well)
6. Observe slow load times (6-7+ seconds)

**Workaround:**

The customer is cleaning up unused organizations to mitigate the issue, but this is not a sustainable long-term solution for deployments that legitimately need many organizations as is the case with this customer as well.

### Relevant Log Output

```shell
Please access the images via ticket 4943.
```

### Expected Behavior

Coder should not induce latency if a user is part of a lot of organizations.

### Steps to Reproduce

1. Create a Coder deployment
2. Create 100+ organizations
3. Add a user to all organizations
4. Log in as that user
5. Navigate to the Workspaces page (the user should have atleast 3-4 workspaces although the latency is noticeable with just 1 workspace as well)
6. Observe slow load times (6-7+ seconds)

### Environment

- Coder version: 2.28.8 (upgrade from 2.24.x)
- Number of organizations: 108
- Database: Migrations successful (schema v393)

### Additional Context

_No response_