Unassigned (no agency) non-admin users previously saw ALL campaigns due to
a truthiness check that treated None agency_id as "no filter". This was a
security bug — they should see NO campaigns and be blocked from creating them.
Backend: Add _NO_AGENCY sentinel to distinguish "no filter" from "no agency",
add early-returns at all 5 list/analytics endpoints, fix _check_campaign_access
to explicitly reject unassigned users, and block campaign creation with 403.
Frontend: Add isUnassigned boolean to UserContext, show informational empty
state on Campaigns view, and reinforce readOnly for defense-in-depth.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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