neotoma has tenant isolation gap in relationship query endpoints

Summary

## Summary

The `/list_relationships` and `/retrieve_graph_neighborhood` endpoints call `getAuthenticatedUserId` (confirming a valid session exists) but do not pass the resolved user ID into the Supabase query as an `.eq("user_id", userId)` filter. As a result, queries return rows from all users rather than scoping to the authenticated caller's data.

## Affected code

**`/list_relationships`** (`src/actions.ts`): - Calls `getAuthenticatedUserId` but does not apply `.eq("user_id", userId)` to the relationships query - Uses `.or()` string interpolation for entity ID matching without input validation

**`/retrieve_graph_neighborhood`** (`src/actions.ts`): - Same pattern: auth resolved, user ID not applied to query filter

## Affected versions

v0.13.0

## Prerequisites

1. A valid authentication token for the Neotoma instance (attacker must have a legitimate account on the same instance) 2. A known entity ID belonging to another user (~96 bits of entropy — brute-force not practical)

An unauthenticated caller is rejected at the auth middleware layer. The gap requires a second user account on the instance.

## Impact

An authenticated user with a known cross-user entity ID can retrieve relationship edges and graph neighborhood data belonging to another user. No write capability is exposed.

## Severity

Low under current conditions — no multi-tenant deployments exist. Escalates to Medium the moment two or more user accounts share an instance.

## Remediation

1. Add `.eq("user_id", userId)` to all Supabase queries in both handlers 2. Validate entity ID inputs with `isNeotomaEntityId` before query construction 3. Replace `.or()` string interpolation with separate scoped `.eq()` calls

Fix tracked in #365 (list_relationships) and #366 (retrieve_graph_neighborhood). Gate gap tracked in #372.