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>

modules/hostmatcher/http.go

@@ -5,8 +5,10 @@ package hostmatcher
 
 import (
 	"context"
+	"crypto/tls"
 	"fmt"
 	"net"
+	"net/http"
 	"net/url"
 	"syscall"
 	"time"
@@ -63,3 +65,17 @@ func NewDialContext(usage string, allowList, blockList *HostMatchList, proxy *ur
 		return dialer.DialContext(ctx, network, addrOrHost)
 	}
 }
+
+// NewHTTPTransport builds an http.Transport that validates the request target against the allow/block
+// lists on the direct-dial path (DialContext). When an HTTP proxy is configured the proxy resolves and
+// dials the target itself, so restricting the proxied target is the proxy server's responsibility, not
+// Gitea's. proxyFunc selects the proxy URL per request (the http.Transport.Proxy selector, e.g.
+// proxy.Proxy()); proxyURLFixed is the fixed proxy address the dialer must always permit; tlsConfig may
+// be nil. blockList may be nil for callers that only maintain an allow-list.
+func NewHTTPTransport(usage string, allowList, blockList *HostMatchList, proxyFunc func(*http.Request) (*url.URL, error), proxyURLFixed *url.URL, tlsConfig *tls.Config) *http.Transport {
+	return &http.Transport{
+		TLSClientConfig: tlsConfig,
+		Proxy:           proxyFunc,
+		DialContext:     NewDialContext(usage, allowList, blockList, proxyURLFixed),
+	}
+}