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/fetcher/worker_findatender.go

@@ -12,6 +12,10 @@ import (
 	"git.avsdev.uk/Go/job-search/internal/util"
 )
 
+// findATenderSource is the Source of the opportunities Find a Tender's
+// converter makes, and so of the ones the Find a Tender source fetches.
+const findATenderSource = "FindATender"
+
 // runFindATender polls Find a Tender every configured interval until ctx
 // is cancelled, passing what each run fetches to save.
 func runFindATender(
@@ -27,7 +31,7 @@ func runFindATender(
 
 	worker := util.Worker{
 		Logger:   tagged(logger),
-		Name:     "FindATender",
+		Name:     findATenderSource,
 		Interval: conf.Interval,
 		Task: func(ctx context.Context, since time.Time) error {
 			opportunities, err := fetchFindATender(ctx, logger, conf, since)