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.go

@@ -0,0 +1,52 @@
+package cli
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"os"
+
+	"git.avsdev.uk/Go/job-search/internal/config"
+	"git.avsdev.uk/Go/job-search/internal/store"
+)
+
+// ErrNoDatabase is returned by OpenConfiguredStore when database.file isn't
+// set.
+var ErrNoDatabase = errors.New("database.file is not set")
+
+// OpenConfiguredStore opens the store at database.file, or returns
+// ErrNoDatabase if none is configured. A missing file or a database that
+// needs migrating fails with a message saying how to create or migrate it; a
+// missing file isn't created, as store.Open would. The caller must close the
+// store.
+func OpenConfiguredStore(
+	ctx context.Context, conf *config.JobSearchConfig,
+) (*store.Store, error) {
+	file := conf.Database.File
+	if file == "" {
+		return nil, ErrNoDatabase
+	}
+
+	_, err := os.Stat(file)
+	if errors.Is(err, os.ErrNotExist) {
+		return nil, fmt.Errorf(
+			"database %s does not exist: run `job-search migrate up --create`",
+			file,
+		)
+	}
+	if err != nil {
+		return nil, fmt.Errorf("database %s: %w", file, err)
+	}
+
+	opportunityStore, err := store.Open(ctx, file)
+	if errors.Is(err, store.ErrNeedsMigration) {
+		return nil, fmt.Errorf(
+			"database %s: %w: run `job-search migrate`", file, err,
+		)
+	}
+	if err != nil {
+		return nil, fmt.Errorf("database %s: %w", file, err)
+	}
+
+	return opportunityStore, nil
+}