feat: add configurable timeline range

migrations/008_timeline_range.sql

@@ -0,0 +1,8 @@
+CREATE TABLE osint.board_timeline_settings (
+  board_id UUID PRIMARY KEY REFERENCES osint.boards(id) ON DELETE CASCADE,
+  range_start DATE NOT NULL,
+  range_end DATE NOT NULL,
+  CHECK (range_end > range_start)
+);
+
+COMMENT ON TABLE osint.board_timeline_settings IS 'Optional authored temporal viewport. Absence means derive the timeline range from dated exhibits.';