[inventory] Add svcs in maintenance to DB

[karencfv]

This isn't ready yet, opening a draft PR because I suck at diesel and in case anyone wants to take a look.

With services in maintenance

$ ./target/debug/omdb db inventory collections show latest --db-url "postgresql://root@[::1]:62900/omicron?sslmode=disable"
<...>

HEALTH MONITOR
    2 SMF services in maintenance at 2026-01-06T06:55:43.045Z
        FMRI                                ZONE   
        svc:/site/fake-service2:default     global 
        svc:/site/fake-service:default      global 

Without services in maintenance

$ ./target/debug/omdb db inventory collections show latest --db-url "postgresql://root@[::1]:48291/omicron?sslmode=disable"
<...>

HEALTH MONITOR
    no data on SMF services in maintenance has been collected

Closes: #9516