Add gated cover byte fetcher (#66)

## Summary

Adds a plain-TLS cover byte fetcher with a destination-class SSRF gate and wires public cover sources through the content-addressed filesystem store.

## Changes

- Resolve hostnames before connecting; refuse non-HTTPS, loopback, private, link-local, unique-local, CGNAT, credentials, and mixed public/private DNS answers.
- Re-check every redirect and resolve/classify again at dial time to close DNS rebinding.
- Reuse `maxBodyBytes`; reject oversized responses and non-image content types before persistence.
- Add generic `Store.GetCover`/`PutCover` source-URL storage while preserving the browser-backed kagane path.
- Keep cover prefetch failures isolated from chapter polling.
- Add observable tests for TLS, no-connection refusals, all refused address classes, redirect blocking, streaming body caps, non-image rejection, content-addressed persistence, DNS rebinding, and poller routing.

## Verification

- `go test -count=1 ./...`
- `go vet ./...`

Both pass. No test touches the live network.

Closes #57

Reviewed-on: #66
Co-authored-by: Sulthan Zaki <sultankiki05@gmail.com>
Co-committed-by: Sulthan Zaki <sultankiki05@gmail.com>

backend/internal/web/cover.go

@@ -65,7 +65,7 @@ func (h *Handler) kaganeCover(w http.ResponseWriter, r *http.Request) {
 		http.NotFound(w, r)
 		return
 	}
-	if !store.IsKaganeCoverContentType(contentType) {
+	if !store.IsCoverContentType(contentType) {
 		log.Printf("kagane cover %s: unexpected content type %q", id, contentType)
 		http.NotFound(w, r)
 		return