hanami new should generate db/.keep when sqlite used

[timriley]

Without this, we see errors like this when booting a new app:

08:12:10 web.1    | ! Unable to load application: Sequel::DatabaseConnectionError: SQLite3::CantOpenException: unable to open database file
08:12:10 web.1    | /Users/tim/.local/share/mise/installs/ruby/3.3.0/lib/ruby/gems/3.3.0/gems/sqlite3-2.0.2-arm64-darwin/lib/sqlite3/database.rb:119:in `open_v2': SQLite3::CantOpenException: unable to open database file (Sequel::DatabaseConnectionError)
08:12:10 web.1    | 	from /Users/tim/.local/share/mise/installs/ruby/3.3.0/lib/ruby/gems/3.3.0/gems/sqlite3-2.0.2-arm64-darwin/lib/sqlite3/database.rb:119:in `initialize'

Per .env, our default sqlite database is DATABASE_URL=sqlite://db/app_name.sqlite.

sqlite will create new database files whenever it needs, but it doesn't create new directories leading to those files, which is why we need db/ to exist.