Docs and restore runbook

Finalise the README so a cold reader can install, configure, back up, and restore from the docs alone.

• Install: the host tools (age and rclone), or the optional container form.
• Label reference (bur.*) and the global bur.toml keys.
• Restore runbook: step by step, including fetching the age private key from Bitwarden, and the live-restore procedure (docker compose stop <app-service>, restore, then start; never docker compose down).
• Supported images: official postgres:* and *-alpine are confirmed; other images are supported only if they pass bur check.
• Alerting setup: creating a Discord channel webhook and the env var that wires it up.
• R2 setup: the bucket, the scoped token, and the padded lifecycle window (roughly 3 to 4 times N times the cadence), with the reason for padding it: an outage must not expire the off-site copies down to zero.
• Optional and deferred: a Dockerfile or container form. Host install is equally valid, so it isn't a v1 requirement.

Acceptance

• a cold reader can install and configure labels from the docs alone.
• the restore runbook covers key retrieval and the stop-the-app live-restore procedure.
• the label reference, bur.toml keys, supported images, alerting setup, and the lifecycle-padding reason are all documented.