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/migrations/0007_covers.sql

@@ -0,0 +1,8 @@
+-- Kagane cover bytes belong in their own table so image blobs never enter the
+-- series queries that drive the latest-chapter poller.
+CREATE TABLE covers (
+  image_id    text PRIMARY KEY,
+  body        bytea NOT NULL,
+  content_type text NOT NULL,
+  fetched_at  timestamptz NOT NULL DEFAULT now()
+);

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).

backend/internal/store/store_test.go

@@ -1231,3 +1231,30 @@ func TestTwoReadersShareOneSeriesWithIndependentProgress(t *testing.T) {
 		t.Fatalf("due after one Reader left = %+v, want the series still polled", due)
 	}
 }
+func TestKaganeCoverPersistsAcrossReopen(t *testing.T) {
+	url := pgtest.URL(t)
+	first, err := Open(url, testOwner)
+	if err != nil {
+		t.Fatalf("Open: %v", err)
+	}
+	body := []byte("stored-cover")
+	if err := first.PutKaganeCover("019fe11a-84c3-7fc3-a84b-88787374b617", body, "image/webp"); err != nil {
+		t.Fatalf("PutKaganeCover: %v", err)
+	}
+	if err := first.Close(); err != nil {
+		t.Fatalf("close first store: %v", err)
+	}
+
+	second, err := Open(url, testOwner)
+	if err != nil {
+		t.Fatalf("reopen: %v", err)
+	}
+	defer second.Close()
+	got, contentType, ok, err := second.GetKaganeCover("019fe11a-84c3-7fc3-a84b-88787374b617")
+	if err != nil {
+		t.Fatalf("GetKaganeCover: %v", err)
+	}
+	if !ok || !bytes.Equal(got, body) || contentType != "image/webp" {
+		t.Fatalf("stored cover = (%q, %q, %v), want (%q, image/webp, true)", got, contentType, ok, body)
+	}
+}