feat(t-paliad-171): migration 075 — project_events.timeline_kind opt-in column

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.

internal/db/migrations/075_project_events_timeline_kind.down.sql

@@ -0,0 +1,6 @@
+-- t-paliad-171 down — drop the SmartTimeline opt-in column.
+
+DROP INDEX IF EXISTS paliad.project_events_timeline_kind_idx;
+
+ALTER TABLE paliad.project_events
+    DROP COLUMN IF EXISTS timeline_kind;

internal/db/migrations/075_project_events_timeline_kind.up.sql

@@ -0,0 +1,32 @@
+-- t-paliad-171 — SmartTimeline Slice 1.
+-- Add the `timeline_kind` opt-in column to paliad.project_events so a
+-- subset of audit rows can surface as timeline content. Existing rows
+-- stay NULL (audit-only) and are filtered out of the SmartTimeline
+-- read path; new write paths (custom milestone, counterclaim_created
+-- in later slices) set the column on insert.
+--
+-- Value space (enforced in code, not via CHECK — see
+-- internal/services/projection_service.go):
+--   'milestone'        — structural event worth pinning to the timeline
+--                        (counterclaim_filed, third_party_intervened,
+--                         party_amendment, our_side_changed, scope_change)
+--   'custom_milestone' — free-text user-added event ("Eigener Meilenstein")
+--   NULL               — audit only (default, all existing rows)
+--
+-- Design ref: docs/design-smart-timeline-2026-05-08.md §2.2.
+
+ALTER TABLE paliad.project_events
+    ADD COLUMN IF NOT EXISTS timeline_kind text NULL;
+
+COMMENT ON COLUMN paliad.project_events.timeline_kind IS
+    'When non-NULL, this audit event also surfaces as a SmartTimeline '
+    'milestone. NULL keeps the row audit-only. See '
+    'internal/services/projection_service.go for the value space.';
+
+-- Partial index — the SmartTimeline read path filters on
+-- (project_id, timeline_kind IS NOT NULL); making the index partial
+-- keeps it tiny (most rows stay audit-only) while still serving the
+-- common lookup.
+CREATE INDEX IF NOT EXISTS project_events_timeline_kind_idx
+    ON paliad.project_events (project_id, timeline_kind)
+    WHERE timeline_kind IS NOT NULL;