fix(db): in_reply_to_id FK ON DELETE SET NULL — deleting a thought no longer blocks if it has replies

crates/adapters/postgres/migrations/010_fix_reply_fk_on_delete.sql

@@ -0,0 +1,11 @@
+-- Change in_reply_to_id FK from RESTRICT (default) to SET NULL.
+-- Previously, deleting a thought that had replies raised a FK violation.
+-- With SET NULL, deleting a thought orphans its replies (they survive but
+-- lose their parent reference), which is the correct semantic for a
+-- threaded social app.
+ALTER TABLE thoughts
+    DROP CONSTRAINT IF EXISTS thoughts_in_reply_to_id_fkey;
+
+ALTER TABLE thoughts
+    ADD CONSTRAINT thoughts_in_reply_to_id_fkey
+    FOREIGN KEY (in_reply_to_id) REFERENCES thoughts(id) ON DELETE SET NULL;