Phase 3: backfill + NOT NULL tightening where safe (DDL)

Phase 3 DDL (db-pluto-cc): backfill actor columns where derivable (from *Archive snapshots / userAuditLogLegacy), then tighten NOT NULL on createdBy for tables where every live row is populated. Runs after phase 2 so new rows are already populated. Leaves NULL where actor is genuinely unknown (pre-audit history). EVO-57 audit-trail schema program; plan authored by coder-pluto-cc from PLUTO-580 gap report (db-pluto-cc). Elazar-gated before DDL.