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/web/web.go

@@ -52,8 +52,7 @@ type Handler struct {
 	httpClient *http.Client
 	// covers proxies kagane cover images, which no browser can load directly.
 	// Nil disables the endpoint — see CoverFetcher.
-	covers     CoverFetcher
-	coverCache coverCache
+	covers CoverFetcher
 }
 
 // listView is what every list-rendering template receives.