chore: improve build deadline code (#19203)

- Adds/improves a lot of comments to make the autostop calculation code
clearer
- Changes the behavior of the enterprise template schedule store to
match the behavior of the workspace TTL endpoint when the new TTL is
zero
- Fixes a bug in the workspace TTL endpoint where it could unset the
build deadline, even though a max_deadline was specified
- Adds a new constraint to the workspace_builds table that enforces the
deadline is non-zero and below the max_deadline if it is set
- Adds CHECK constraint enum generation to scripts/dbgen, used for
testing the above constraint
- Adds Dean and Danielle as CODEOWNERS for the autostop calculation code

coderd/provisionerdserver/provisionerdserver.go

@@ -1866,8 +1866,9 @@ func (s *server) completeWorkspaceBuildJob(ctx context.Context, job database.Pro
 			Database:                    db,
 			TemplateScheduleStore:       templateScheduleStore,
 			UserQuietHoursScheduleStore: *s.UserQuietHoursScheduleStore.Load(),
-			Now:                         now,
-			Workspace:                   workspace.WorkspaceTable(),
+			// `now` is used below to set the build completion time.
+			WorkspaceBuildCompletedAt: now,
+			Workspace:                 workspace.WorkspaceTable(),
 			// Allowed to be the empty string.
 			WorkspaceAutostart: workspace.AutostartSchedule.String,
 		})