fix(self-host): clarify reinstall recovery

[Uarmagan]

Summary

This PR improves the local self-host reinstall path and makes the self-host README easier to use when someone is confused or blocked.

What changed

• Detects existing Compass Docker volumes when ~/compass/.env is missing and stops before creating a new .env that could lock the user out of old data.
• Keeps older self-host installs working by falling back to the existing Mongo root password when MONGO_REPLICA_SET_KEY is absent.
• Reworks self-host/README.md into a troubleshooting-focused guide for install, helper commands, Docker volumes, .env changes, update/rebuild behavior, ports, and Google Calendar limitations.
• Adds a pointer from the long self-hosting guide to the self-host runtime README for quick local installer troubleshooting.

Why

Re-running the installer after deleting ~/compass but leaving Docker volumes behind produced a confusing Docker failure. The installer now explains what happened, why it stopped, and which recovery path to choose. The README now mirrors that support flow so users have a clearer place to self-serve when Docker, volumes, or env files get confusing.

Validation

• sh -n self-host/install.sh
• Simulated reinstall with existing Docker volumes and missing ~/compass/.env
• docker compose --env-file <older-env-without-MONGO_REPLICA_SET_KEY> -f self-host/docker-compose.yml config
• Checked README/doc links resolve locally
• Checked documented helper commands match self-host/compass
• git diff --check --cached

Note: Biome reports these docs and shell files are ignored by the repo config, so there was no Biome file check to apply here.