Persist kagane covers in Postgres (#49)

Closes #43

Persist kagane cover bytes in a dedicated Postgres covers table keyed by image ID. The web handler reads storage before the browser, writes validated fetches through, and no longer keeps an in-process cover cache. Added migration, store persistence tests including reopen, handler coverage for stored/miss/rejected paths, and corrected repository guidance.

Verification:
- go test ./...
- CGO_ENABLED=0 go build ./...

Reviewed-on: #49
Co-authored-by: Sulthan Zaki <sultankiki05@gmail.com>
Co-committed-by: Sulthan Zaki <sultankiki05@gmail.com>

backend/internal/store/store.go

@@ -529,6 +529,37 @@ func scanSeries(scan func(...any) error) (Series, error) {
 // Close releases the underlying database handle.
 func (s *Store) Close() error { return s.db.Close() }
 
+// GetKaganeCover returns one persisted cover. Missing covers are reported with
+// ok=false rather than as an error so the web handler can fetch them once.
+func (s *Store) GetKaganeCover(imageID string) ([]byte, string, bool, error) {
+	var (
+		body        []byte
+		contentType string
+	)
+	err := s.db.QueryRow(
+		`SELECT body, content_type FROM covers WHERE image_id = $1`, imageID,
+	).Scan(&body, &contentType)
+	if errors.Is(err, sql.ErrNoRows) {
+		return nil, "", false, nil
+	}
+	if err != nil {
+		return nil, "", false, fmt.Errorf("get kagane cover %q: %w", imageID, err)
+	}
+	return body, contentType, true, nil
+}
+
+// PutKaganeCover persists one fetched cover. Image ids are immutable, so a
+// later fetch cannot replace the bytes already made durable.
+func (s *Store) PutKaganeCover(imageID string, body []byte, contentType string) error {
+	if _, err := s.db.Exec(`
+		INSERT INTO covers (image_id, body, content_type)
+		VALUES ($1, $2, $3)
+		ON CONFLICT (image_id) DO NOTHING`, imageID, body, contentType); err != nil {
+		return fmt.Errorf("put kagane cover %q: %w", imageID, err)
+	}
+	return nil
+}
+
 // List returns every bookmark of one reader, newest activity first.
 // Series-owned fields are joined in, so each Bookmark reads back whole and
 // flat (ADR-0004).