fix: correct chat_histories FK to reference projects(guid)

Fixes #2 — project_id on chat_histories was referencing projects(id)
(bigserial) instead of projects(guid) (uuid). Added migration 019 to
repair existing deployments and corrected 018 for fresh installs.

migrations/019_fix_chat_histories_fk.sql

@@ -0,0 +1,11 @@
+-- Migration: 019_fix_chat_histories_fk
+-- Fix the project_id foreign key on chat_histories to reference projects(guid)
+-- instead of projects(id). This corrects the initial migration (018) which
+-- used the wrong column — projects.id is a bigserial, projects.guid is the UUID.
+
+ALTER TABLE chat_histories
+    DROP CONSTRAINT IF EXISTS chat_histories_project_id_fkey;
+
+ALTER TABLE chat_histories
+    ADD CONSTRAINT chat_histories_project_id_fkey
+    FOREIGN KEY (project_id) REFERENCES projects(guid) ON DELETE SET NULL;