Get Graph Stats

Get Neural Memory graph statistics for specified context.

Issue #82: Context-scoped stats. Issue #383: Authorization is delegated to PermissionService. Previously this endpoint hard-filtered by user_id == caller, hiding shared-context memories authored by other workspace members. Now:

- Shared context (``is_private = false``): any workspace member sees
  the full graph for the context.
- Private context (``is_private = true``): only the private-context
  creator can access the graph for that context. The creator-only
  rule matches ``can_access_memory`` (returns ``False`` for
  non-creators) and ``check_context_access`` (raises 403 for
  non-creators). This endpoint deliberately **remaps that denial
  to 404**, unifying with the cross-workspace-probe shape so
  neither path leaks existence — an intentional divergence from
  ``check_context_access``'s 403 status code, not from its
  authorization semantics.
- Cross-workspace probes and private-context access by non-creators
  both surface as 404 (CWE-639 / OWASP A01 uniform disclosure)
  to avoid existence leakage.

Args: context_id: Optional context ID. When absent, returns an empty graph (no scope = no visible edges).

Returns: Graph statistics including node/edge counts, top connections, etc.