Copyparty: require your own password

[jackspiering]

Description

Copyparty shipped the account admin with the password changeme and read-write access to the shared host folder. It was the last working default credential in any compose or .env file after #367 and #368.

• compose.yaml now reads the password as ${COPYPARTY_PASSWORD:?Set COPYPARTY_PASSWORD in .env}.
• .env has a # Required: comment and an empty COPYPARTY_PASSWORD=.
• The README updates "Before you start" and "First run", and adds an "Upgrading" section for users who changed the password in compose.yaml or kept changeme.

Related Issues

• None.

Verification

• docker compose config --quiet passes with a dummy value and stops with required variable COPYPARTY_PASSWORD is missing a value without one. The rendered account line is admin:<password>.
• Upstream's [accounts] parser splits on the first colon, so the format is valid. The README asks for letters and digits only, which openssl rand -hex 32 satisfies.
• rumdl check --config .markdownlint.yml . (0.2.78) and git diff --check are clean.
• Not run: the stack through the Tailnet. This is a config-only change.

Checklist

• I have performed a self-review of my code and followed the templates structure.
• I have added verification that the stack works as expected.
• I have updated necessary documentation (e.g. frontpage README.md ).
• I have selected the correct label(s) for this PR.

Additional Context

From an external review of the repository (section 3.1). The placeholder host path in the Copyparty stack is a separate item and is not changed here.