GHSA-59h8-w5q6-mfmp

## Summary

The POST handler for `/realtime/v1/streams/:runId/:streamId` has no authentication. Any entity that knows or guesses a run friendlyId can inject arbitrary data into its realtime stream.

## Vulnerability Details

**File:** `apps/webapp/app/routes/realtime.v1.streams.$runId.$streamId.ts`

The `action` handler (line 17) has no auth wrapper. The code comment says: "Plain action for backwards compatibility with older clients that don't send auth headers."

The run lookup at line 29 uses `where: { friendlyId: runId }` with NO environment scoping (`runtimeEnvironmentId` is not checked), so production runs are accessible.

Run friendlyIds follow predictable patterns (e.g., `run_1234abcd`).

## Steps to Reproduce

```bash # No authentication required curl -X POST "http://localhost:8030/realtime/v1/streams/run_KNOWN_ID/stream_1" -H "Content-Type: application/json" -d '{"injected": "data"}' ```

## Impact

Unauthenticated data injection into any run realtime stream. Cross-environment access (no scoping).