Adds a nullable text column on paliad.project_events so a subset of
audit rows can opt into surfacing as SmartTimeline content. Existing
rows stay NULL (audit-only); the partial index keeps the lookup tiny
because the SmartTimeline read filter is the indexed predicate.
Value space (enforced in code in internal/services/projection_service.go):
'milestone' — structural event (counterclaim_filed, ...)
'custom_milestone' — free-text "Eigener Meilenstein"
NULL — audit only (default)
Design ref: docs/design-smart-timeline-2026-05-08.md §2.2.
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