v0.9 ticket 02. Campaigns now carry an exploration seed set (seed
personas × seed goals — the comparability unit for exploratory
evaluation) and an optional budget override, stored as JSON columns
isomorphic to plan. Empty seeds normalize to null, marking the campaign
as opted out of exploration. resolve_budget merges per-field overrides
into platform defaults; enforcement stays server-side. The create form
gains seed lists and budget inputs (minutes → seconds), submitting null
when left empty.
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