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_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)
+	}
+}