docs: document missing ENV variables for mailing and assets

[rewritten]

Summary

• Adds missing MAILER_SENDER to README ENV table and .env.example — the sender address for all Devise emails (fixes Document MAILER_SENDER #827)
• Fixes MAIL_LINK_PROTOCOL → MAIL_LINK_PROTO in README to match the actual variable name used in the code and .env.example
• Adds RAILS_ASSET_HOST and ASSETS_PRECOMPILE which were used in config/environments/production.rb but undocumented

Test plan

• Review README ENV table for accuracy and completeness
• Confirm .env.example contains MAILER_SENDER with a sensible placeholder

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR updates repository documentation to better reflect the environment variables the Rails app actually reads for production assets and outbound email configuration.

Changes:

• Documents RAILS_ASSET_HOST and ASSETS_PRECOMPILE in the README ENV table.
• Fixes the README env var name to MAIL_LINK_PROTO to match production config.
• Adds MAILER_SENDER to both the README and .env.example for Devise sender configuration.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
README.md	Adds/mends ENV variable documentation for asset hosting/precompile and mailer URL/sender settings.
.env.example	Adds MAILER_SENDER example value for local/dev configuration.

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