Add (Postgres + SQLite) support to matrix-mautrix-hangouts bridge

I don't use this bridge, so this is completely untested.

roles/matrix-bridge-mautrix-hangouts/tasks/setup_install.yml

@@ -8,6 +8,26 @@
       The matrix-bridge-mautrix-hangouts role needs to execute before the matrix-synapse role.
   when: "matrix_synapse_role_executed|default(False)"
 
+- block:
+  - name: Check if an SQLite database already exists
+    stat:
+      path: "{{ matrix_mautrix_hangouts_sqlite_database_path_local }}"
+    register: matrix_mautrix_hangouts_sqlite_database_path_local_stat_result
+
+  - name: Fail if an SQLite database already exists when using Postgres
+    fail:
+      msg: >-
+        matrix_mautrix_hangouts_database_engine has been set to `postgres` (which is our new default now).
+        However, we've discovered an existing SQLite database in {{ matrix_mautrix_hangouts_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_mautrix_hangouts_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_mautrix_hangouts_sqlite_database_path_local }} postgres_connection_string_variable_name=matrix_mautrix_hangouts_postgres_connection_string'`)
+        3. Re-run the playbook (`ansible-playbook -i inventory/hosts setup.yml --tags=setup-all,start`)
+    when: "matrix_mautrix_hangouts_sqlite_database_path_local_stat_result.stat.exists"
+  when: "matrix_mautrix_hangouts_database_engine == 'postgres'"
+
 - name: Ensure Mautrix Hangouts image is pulled
   docker_image:
     name: "{{ matrix_mautrix_hangouts_docker_image }}"