Made each source start from its newest stored release, with --since as an override

Without --since, fetcher service and job-search serve start each source
from the newest release stored for it (the new store.LatestReleaseDate,
by Source), so a restart carries on where it left off instead of fetching
the last day again; with nothing stored, the client's default still
applies. --since overrides it for every source. The Find a Tender source's
name is now a constant, as the Source its opportunities are stored with.

The README's guide to adding a source is brought up to date with the
current worker, start and config type names.

internal/cli/fetch/service.go

@@ -79,7 +79,8 @@ func commandService(opts *cli.Options) *cobra.Command {
 		[]string{
 			time.DateOnly, time.DateTime,
 		},
-		"contract updates published since YYYY-MM-DD[ hh:mm:ss]",
+		"contract updates published since YYYY-MM-DD[ hh:mm:ss] "+
+			"(default: each source's newest stored release, or the last day)",
 	)
 	cmd.Flags().Bool("foreground", false, "run in the foreground")