Remove passlib dependency by making matrix-media-repo datastore IDs user-provided

These IDs were incorrectly auto-derived from matrix_homeserver_generic_secret_key,
which is meant for secrets that are OK to change. Datastore IDs are static
identifiers that must never change after first use.

The playbook now requires users to explicitly set matrix_media_repo_datastore_file_id
(and matrix_media_repo_datastore_s3_id when S3 is enabled) in vars.yml, with
validation that fails early if they are missing.

This was the last usage of passlib, which is now removed from prerequisites.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/prerequisites.md

@@ -23,8 +23,6 @@ We will be using `example.com` as the domain in the following instruction. Pleas
 
 - [Ansible](http://ansible.com/) program. It's used to run this playbook and configures your server for you. Take a look at [our guide about Ansible](ansible.md) for more information, as well as [version requirements](ansible.md#supported-ansible-versions) and alternative ways to run Ansible.
 
-- [passlib](https://passlib.readthedocs.io/en/stable/index.html) Python library. See [this official documentation](https://passlib.readthedocs.io/en/stable/install.html#installation-instructions) for an instruction to install it. On most distros, you need to install some `python-passlib` or `py3-passlib` package, etc.
-
 - [`git`](https://git-scm.com/) as the recommended way to download the playbook. `git` may also be required on the server if you will be [self-building](self-building.md) components.
 
 - [`just`](https://github.com/casey/just) for running `just roles`, `just update`, etc. (see [`justfile`](../justfile)), although you can also run these commands manually. Take a look at this documentation for more information: [Running `just` commands](just.md).