Fail if appservice-discord wants Postgres, but has leftover SQLite data

roles/matrix-bridge-appservice-discord/tasks/setup_install.yml

@@ -1,5 +1,21 @@
 ---
 
+- block:
+  - name: Check if an SQLite database already exists
+    stat:
+      path: "{{ matrix_appservice_discord_data_path }}/{{ matrix_appservice_discord_database_filename_name }}"
+    register: matrix_appservice_discord_stat_sqlite_db
+
+  - name: Fail if an SQLite database already exists when using Postgres
+    fail:
+      msg: >-
+        matrix_appservice_discord_database_engine has been set to `postgres` (which is our new default now).
+        However, we've discovered an existing SQLite database in {{ matrix_appservice_discord_data_path }}/{{ matrix_appservice_discord_database_filename_name }}.
+        It appears that you've been using this bridge with the SQLite engine until now.
+        To continue using SQLite, opt into it explicitly: add `matrix_appservice_discord_database_engine: sqlite` to your vars.yml file.
+        To migrate to Postgres: TODO - migration instructions here.
+  when: "matrix_appservice_discord_database_engine == 'postgres'"
+
 - name: Ensure Appservice Discord image is pulled
   docker_image:
     name: "{{ matrix_appservice_discord_docker_image }}"