feat: default to SQLite engine, disable Mongo service by default in compose

Mongo services in both compose files are now commented out (with instructions
to re-enable) since DB_ENGINE now defaults to sqlite - no separate database
container required out of the box. db-data volume is mounted into the app
service (not just mongo) so it can hold the SQLite file too.

Also fixes a require-order bug in tests/utils/express-app.js: routers were
required (eagerly instantiating DB singletons via dbFactory) before the test
env file was loaded, so DB_ENGINE from .env.test was silently never applied -
it only worked before because the old hardcoded default happened to be mongo.

.env.example

@@ -3,17 +3,19 @@ DOCKER=true
 
 # Database Engine: Choose between "mongo" and "sqlite", this specifies where file/folder/user METADATA is stored.
 # This is separate from DB_TYPE below, which controls where the raw file BYTES are stored.
-# mongo = MongoDB (default)
-# sqlite = embedded SQLite file, no separate database server needed
-DB_ENGINE=mongo
+# sqlite = embedded SQLite file, no separate database server needed (default)
+# mongo = MongoDB - requires uncommenting the mongo service in the compose file
+DB_ENGINE=sqlite
 
-# MongoDB URL: Connection string for your MongoDB database (only used when DB_ENGINE=mongo)
-# Note: if using the compose file provided, the connection string should be as follows:
-MONGODB_URL=mongodb://username:password@mongo:27017/mydrive?authSource=admin
+# MongoDB URL (only used when DB_ENGINE=mongo): Connection string for your MongoDB database.
+# Note: if using the compose file provided, uncomment the mongo service there first,
+# then the connection string should be as follows:
+# MONGODB_URL=mongodb://username:password@mongo:27017/mydrive?authSource=admin
 
 # SQLite DB Path (only used when DB_ENGINE=sqlite): path to the SQLite database file.
-# Should live inside a persisted volume (e.g. the same one used for FS_DIRECTORY) so it survives restarts.
-SQLITE_DB_PATH=/data/mydrive.db
+# Should live inside a persisted volume so it survives restarts.
+# Note: if using the compose file provided, this should point into the db-data volume:
+SQLITE_DB_PATH=/db-data/mydrive.db
 
 # Database Type: Choose between "fs" and "s3", this specifies where the files will be stored.
 # fs = Filesystem