Add (Postgres + SQLite) support to matrix-reminder-bot

This has been tested and appears to work.

roles/matrix-bot-matrix-reminder-bot/tasks/setup_install.yml

@@ -1,5 +1,25 @@
 ---
 
+- block:
+  - name: Check if an SQLite database already exists
+    stat:
+      path: "{{ matrix_bot_matrix_reminder_bot_sqlite_database_path_local }}"
+    register: matrix_bot_matrix_reminder_bot_sqlite_database_path_local_stat_result
+
+  - name: Fail if an SQLite database already exists when using Postgres
+    fail:
+      msg: >-
+        matrix_bot_matrix_reminder_bot_database_engine has been set to `postgres` (which is our new default now).
+        However, we've discovered an existing SQLite database in {{ matrix_bot_matrix_reminder_bot_sqlite_database_path_local }}.
+        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_bot_matrix_reminder_bot_database_engine: sqlite` to your vars.yml file and re-run this same command.
+        Alternatively, to migrate your existing SQLite database to Postgres:
+        1. Stop all services (`ansible-playbook -i inventory/hosts setup.yml --tags=stop`)
+        2. Import the SQLite database into Postgres (`ansible-playbook -v -i inventory/hosts setup.yml --tags=import-generic-sqlite-db --extra-vars='sqlite_database_path={{ matrix_bot_matrix_reminder_bot_sqlite_database_path_local }} postgres_connection_string_variable_name=matrix_bot_matrix_reminder_bot_database_connection_string'`)
+        3. Re-run the playbook (`ansible-playbook -i inventory/hosts setup.yml --tags=setup-all,start`)
+    when: "matrix_bot_matrix_reminder_bot_sqlite_database_path_local_stat_result.stat.exists"
+  when: "matrix_bot_matrix_reminder_bot_database_engine == 'postgres'"
+
 - name: Ensure matrix-reminder-bot paths exist
   file:
     path: "{{ item.path }}"