Made the fetcher save what it fetches, with fetcher.WithStore

With WithStore, the Find a Tender and mock workers save each run's
opportunities with SaveOpportunities and log what changed; a failed save
fails the run, so the checkpoint stays put and the next run retries.
Without a store nothing is persisted. The mock's opportunities are saved
with Source "FTSMock" rather than the converter's "FindATender".

fetcher service and job-search serve open the store at database.file
(cli.OpenConfiguredStore), or warn and run without one. A missing file
isn't created, and a database needing migration says to run job-search
migrate.

Fixed upsertOpportunity, which never wrote the source column, so every
saved opportunity had an empty Source.

internal/cli/store_test.go

@@ -0,0 +1,66 @@
+package cli
+
+import (
+	"context"
+	"errors"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"git.avsdev.uk/Go/job-search/internal/config"
+	"git.avsdev.uk/Go/job-search/internal/store"
+)
+
+// No database.file means no store, reported as ErrNoDatabase.
+func TestOpenConfiguredStoreNone(t *testing.T) {
+	opportunityStore, err := OpenConfiguredStore(
+		context.Background(), &config.JobSearchConfig{},
+	)
+	if opportunityStore != nil || !errors.Is(err, ErrNoDatabase) {
+		t.Errorf(
+			"OpenConfiguredStore = %v, %v; want nil, ErrNoDatabase",
+			opportunityStore, err,
+		)
+	}
+}
+
+// A missing database isn't created; one that needs migrating says how to
+// migrate it; a migrated one opens.
+func TestOpenConfiguredStore(t *testing.T) {
+	conf := &config.JobSearchConfig{}
+	conf.Database.File = filepath.Join(t.TempDir(), "jobsearch.db")
+
+	_, err := OpenConfiguredStore(context.Background(), conf)
+	if err == nil || !strings.Contains(err.Error(), "migrate up --create") {
+		t.Fatalf("missing: err = %v; want a hint to create it", err)
+	}
+	_, statErr := os.Stat(conf.Database.File)
+	if !errors.Is(statErr, os.ErrNotExist) {
+		t.Fatal("the missing database file was created")
+	}
+
+	// An empty file is a database at version 0.
+	if err := os.WriteFile(conf.Database.File, nil, 0o600); err != nil {
+		t.Fatal(err)
+	}
+	_, err = OpenConfiguredStore(context.Background(), conf)
+	if !errors.Is(err, store.ErrNeedsMigration) ||
+		!strings.Contains(err.Error(), "run `job-search migrate`") {
+		t.Fatalf("unmigrated: err = %v; want ErrNeedsMigration and a hint", err)
+	}
+
+	migrated, err := store.Open(
+		context.Background(), conf.Database.File, store.WithAutoMigrate(),
+	)
+	if err != nil {
+		t.Fatal(err)
+	}
+	migrated.Close()
+
+	opportunityStore, err := OpenConfiguredStore(context.Background(), conf)
+	if err != nil {
+		t.Fatalf("migrated: err = %v", err)
+	}
+	opportunityStore.Close()
+}