Cherry-picks the one genuinely new fix from #169 after confirming the
rest of that PR is already covered on main (C1/C3/C5 by wsAuth group,
C6 by #94+#119 SSRF blocklist, C4 ownership by existing WHERE filter).
Pre-existing middleware (WorkspaceAuth on /workspaces/:id/* sub-routes)
proves the caller owns the :id path param. But the body field
source_id was never validated — a workspace authenticated for its own
/activity endpoint could still attribute logs to a different workspace
by setting source_id=<foreign UUID>. Rejected with 403 now.
No schema change, no new middleware. 4-line handler delta. Closes the
only real gap in #169; #169 itself will be closed as superseded.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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