feat: add app status tracking to the backend (#17163)

This does ~95% of the backend work required to integrate the AI work.

Most left to integrate from the tasks branch is just frontend, which
will be a lot smaller I believe.

The real difference between this branch and that one is the abstraction
-- this now attaches statuses to apps, and returns the latest status
reported as part of a workspace.

This change enables us to have a similar UX to in the tasks branch, but
for agents other than Claude Code as well. Any app can report status
now.

cli/testdata/coder_list_--output_json.golden

@@ -69,6 +69,7 @@
         "most_recently_seen": null
       }
     },
+    "latest_app_status": null,
     "outdated": false,
     "name": "test-workspace",
     "autostart_schedule": "CRON_TZ=US/Central 30 9 * * 1-5",