The backup alarm is a dead man's switch: a missed check is itself red #8959

From #8742. Done when:

• [x] The check that backups are fresh runs independently of the backup job it watches, and records each run it makes.
• [x] When no check has run within its interval, home's backups tile and /api/v1 health show red, saying when the last check ran.
• [x] A test: stop the check, advance the clock past its interval, and the tile is red.

Files: the backups status module, home's tile, a test.

Reviewed pull #8980 at its head: ticked The check that backups are fresh runs independently of the backup job it watches, and records each run it makes.; When no check has run within its interval, home's backups tile and /api/v1 health show red, saying when the last check ran.; A test: stop the check, advance the clock past its interval, and the tile is red..