fix(api): validate same-origin before forwarding auth on --paginate

Codex flagged: the --paginate loop rebuilt the next request from the raw
`Link: rel="next"` URL and reattached the bearer token without checking
that the next URL was on the same host. Forgejo emits same-origin next-
links in practice, but a buggy or malicious upstream could redirect us
to a foreign host, at which point the token would leak.

Now the loop:
- url.Parse the Link target.
- Resolve relative URLs against the original base (https://<host>/api/v1).
- Refuse to proceed if the resolved URL's scheme isn't https or its host
  doesn't match `host.Hostname`. The error names both the foreign URL
  and the expected origin so the user can tell why pagination stopped.

Verified: same-origin pagination still works (`--paginate` against
forgejo.zerova.net commits returns 44 across 22 pages).

cmd/api.go

@@ -291,7 +291,22 @@ func runAPI(cmd *cobra.Command, args []string) error {
 		bodies := [][]byte{respBody}
 		nextURL := parseLinkHeaderNext(respHeader.Get("Link"))
 		for nextURL != "" {
-			nextReq, err := http.NewRequest(http.MethodGet, nextURL, nil)
+			// Forgejo emits same-origin next-links in practice, but a buggy
+			// or hostile upstream could redirect us to a foreign host — at
+			// which point we'd leak the bearer token. Validate origin (and
+			// resolve relative URLs against `base`) before forwarding auth.
+			parsedNext, err := url.Parse(nextURL)
+			if err != nil {
+				return fmt.Errorf("invalid Link rel=\"next\" URL %q: %w", nextURL, err)
+			}
+			if !parsedNext.IsAbs() {
+				parsedNext = base.ResolveReference(parsedNext)
+			}
+			if parsedNext.Scheme != "https" || parsedNext.Host != host.Hostname {
+				return fmt.Errorf("paginated next URL %s is not same-origin as https://%s; refusing to forward credentials", parsedNext.String(), host.Hostname)
+			}
+
+			nextReq, err := http.NewRequest(http.MethodGet, parsedNext.String(), nil)
 			if err != nil {
 				return fmt.Errorf("failed to build paginated request: %w", err)
 			}
@@ -311,7 +326,7 @@ func runAPI(cmd *cobra.Command, args []string) error {
 				return err
 			}
 			if pageStatus < 200 || pageStatus >= 300 {
-				return fmt.Errorf("paginated request to %s failed with status %d", nextURL, pageStatus)
+				return fmt.Errorf("paginated request to %s failed with status %d", parsedNext.String(), pageStatus)
 			}
 			bodies = append(bodies, pageBody)
 			nextURL = parseLinkHeaderNext(pageHeader.Get("Link"))