fix: various security fixes (#38406)

Addresses a batch of privately reported security issues, grouped by
area:

- **SSRF** - migration PR-patch/asset fetches, OAuth2 avatar & OpenID
discovery, pull-mirror URL re-validation, and the outbound proxy path.
- **Access-token scope** - prevent scope escalation on token creation;
keep public-only tokens confined (feeds, packages, Actions listings,
star/watch lists, limited/private owners).
- **Access control / disclosure** - go-get default-branch leak, webhook
authorization-header leak, watch clearing on private transitions,
label/attachment scoping.
- **Denial of service** - input bounds for npm dist-tags, Debian control
files, Arch file lists, and SSH keys.

### 📌 Attention for site admins

Not breaking - existing configs keep working - but two changes are worth
a look:

- **New SSRF protection** Outbound requests (migrations, OAuth2 avatars,
OpenID discovery, pull mirrors, proxy path) are now validated against
the allow/block host lists. If your instance legitimately reaches
internal hosts, you may need to add them to
`[security].ALLOWED_HOST_LIST` (and the relevant `ALLOW_LOCALNETWORKS`
settings).
- **Deprecation** `[webhook].ALLOWED_HOST_LIST` is deprecated and will
be removed in a future release. Use `[security].ALLOWED_HOST_LIST`
instead; the old key still works for now.

---------

Co-authored-by: TheFox0x7 <thefox0x7@gmail.com>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Zettat123 <zettat123@gmail.com>

services/migrations/dump.go

@@ -8,7 +8,6 @@ import (
 	"errors"
 	"fmt"
 	"io"
-	"net/http"
 	"net/url"
 	"os"
 	"path/filepath"
@@ -310,7 +309,9 @@ func (g *RepositoryDumper) CreateReleases(_ context.Context, releases ...*base.R
 						}
 						defer rc.Close()
 					} else {
-						resp, err := http.Get(*asset.DownloadURL)
+						// use the migration client so the fetch (including any redirect) is
+						// validated against the migration host allow/block list
+						resp, err := getMigrationHTTPClient().Get(*asset.DownloadURL)
 						if err != nil {
 							return err
 						}
@@ -450,8 +451,10 @@ func (g *RepositoryDumper) handlePullRequest(ctx context.Context, pr *base.PullR
 		}
 
 		// SECURITY: We will assume that the pr.PatchURL has been checked
-		// pr.PatchURL maybe a local file - but note EnsureSafe should be asserting that this safe
-		resp, err := http.Get(u) // TODO: This probably needs to use the downloader as there may be rate limiting issues here
+		// pr.PatchURL maybe a local file - but note EnsureSafe should be asserting that this safe.
+		// Use the migration client so an http(s) PatchURL (and any redirect it follows) is
+		// validated against the migration host allow/block list at dial time.
+		resp, err := getMigrationHTTPClient().Get(u)
 		if err != nil {
 			return err
 		}