feat(properties): add parent_id for sub-property hierarchy

Phase 1 of the parent/child rollup feature. Self-FK on properties
with ON DELETE RESTRICT, plus a CHECK that blocks self-reference at
the DB level. Service-layer helpers (getDescendantIds,
getAncestorIds, assertNoCycle) walk the tree via recursive CTEs and
guard against cycles and cross-company parents. softDeleteProperty
now refuses to delete a property with live children.

No UI yet — readers and roll-up routes land in Phase 2.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

drizzle/0016_property_parent_id.sql

@@ -0,0 +1,3 @@
+ALTER TABLE "properties" ADD COLUMN "parent_id" uuid;--> statement-breakpoint
+ALTER TABLE "properties" ADD CONSTRAINT "properties_parent_id_properties_id_fk" FOREIGN KEY ("parent_id") REFERENCES "public"."properties"("id") ON DELETE restrict ON UPDATE no action;--> statement-breakpoint
+CREATE INDEX "properties_by_parent" ON "properties" USING btree ("company_id","parent_id");