docs(self-host): document Docker/Podman uninstall in SELF_HOSTING.md

## Problem
Reporter cannot uninstall World Monitor. Title typo: Uninsttall. No OS or install path given. A comment said "just destroy the docker containers," which is the Docker path but is not documented as uninstall.

## What already exists
`SELF_HOSTING.md` is the install path for Docker/Podman:
- `docker compose up -d` / `uvx podman-compose up -d`
- `./scripts/run-seeders.sh`
- Redis data lives in the `redis-data` volume and **is lost on `docker compose down -v`** (already stated in the seeder section)
- Rebuild: `docker compose down && docker compose up -d`

There is **no uninstall script** in the repo. That is a docs gap, not a missing binary, unless they installed the desktop AppImage (see #5757 / #5902), which this issue does not mention.

## Scope
Document how to fully remove a **self-hosted Docker/Podman** install. Do not invent a new packaging format.

**In scope**
- A `## Uninstall` section in `SELF_HOSTING.md` that names the commands and what they delete
- `docker compose down -v` (or podman-compose equivalent) plus removal of the clone, `.env`, and `docker-compose.override.yml`
- Explicit warning that `-v` wipes Redis

**Out of scope**
- Desktop AppImage / Tauri uninstall (#5757, #5902)
- A new `scripts/uninstall.sh` unless the docs path is not enough (YAGNI)
- Uninstalling a Vercel/hosted WorldMonitor.app account

## Acceptance
- [ ] `SELF_HOSTING.md` has an Uninstall section with copy-pasteable Docker and Podman commands
- [ ] Commands named: stop stack, remove volumes, remove clone + `.env` + override file
- [ ] Volume-wipe warning is next to the command, not only in the seeder section
- [ ] Desktop/AppImage is pointed at the desktop issues, not faked here

Effort: XS (~0.25d). Docs, not a new script.

Related: #6647 (what self-hosting actually gives you), #5415 (self-host 401).

Read SELF_HOSTING.md and locate the existing Docker/Podman install and rebuild instructions. Add an Uninstall section covering the Docker and podman-compose stop, volume-removal, and clone cleanup steps, with the Redis data warning beside the volume command. Confirm the section is copy-pasteable and points desktop/AppImage users to the referenced desktop issues.

Written by the indexing model from the issue text.