[inventory] Add stale saga information

[karencfv]

As per #9876, this PR adds information about stale sagas to inventory. Any saga that is older than 15 minutes which is in a running or unwinding state.

The 15 minute cutoff is arbitrary, the update status endpoint will only consider a saga older than 1 hour as a health concern, but it would be good to store more data on sagas that are in one of these two states for a shorter time for debugging purposes. I can change this threshold if people don't agree though

A stale saga is present:

$ ./target/debug/omdb db inventory collections show latest
<...>
STALE SAGAS
    Found 1 running or unwinding for longer than 15 minutes
        NAME     ID                                       STATE       CREATOR                                  CURRENT_SEC                              TIME_CREATED                   
        demo     457c4832-1a87-4bd3-a6d1-0e98105b1a5c     running     a4ef738a-1fb0-47b1-9da2-4919c7ec7c7f     a4ef738a-1fb0-47b1-9da2-4919c7ec7c7f     2026-04-13 07:39:27.357491 UTC 

Everything is happy:

$ ./target/debug/omdb db inventory collections show latest

<...>
STALE SAGAS
    No sagas have been running or unwinding for longer than 15 minutes

Closes: #9412

[karencfv]

From the build-and-test (helios) job https://buildomat.eng.oxide.computer/wg/0/details/01KP5DY5F65D03T0G7EHK3D0VX/3HTS7wQUErwzoUUVG6juJt1SdvKCVqdRoL8vOvWb0bSdknVd/01KP5DYFQWBTSMENHD0BFYC31X#S6677

6677	2026-04-14T07:40:41.181Z	error: creating test list failed
6678	2026-04-14T07:40:41.181Z	
6679	2026-04-14T07:40:41.181Z	Caused by:
6680	2026-04-14T07:40:41.181Z	  for `nexus-fm`, command `/work/oxidecomputer/omicron/target/debug/deps/nexus_fm-618b2e4b6bedc71d --list --format terse` aborted with signal 9 (SIGKILL)
6681	2026-04-14T07:40:41.181Z	  --- stdout:
6682	2026-04-14T07:40:41.181Z	
6683	2026-04-14T07:40:41.181Z	  --- stderr:
6684	2026-04-14T07:40:41.181Z	  ld.so.1: nexus_fm-618b2e4b6bedc71d: fatal: libpq.so.5: open failed: No such file or directory
6685	2026-04-14T07:40:41.181Z	
6686	2026-04-14T07:40:41.181Z	  ---

I am legitimately stumped by this. I didn't even touch that crate. The build-and-test (ubuntu-22.04) tests run fine, and the problem isn't showing on main either!!