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>

tests/integration/api_packages_npm_test.go

@@ -208,6 +208,12 @@ func TestPackageNpm(t *testing.T) {
 		test(t, http.StatusNotFound, packageTag2, "1.2")
 		test(t, http.StatusOK, packageTag, packageVersion)
 		test(t, http.StatusOK, packageTag2, packageVersion)
+
+		// an oversized dist-tag body is rejected instead of being read unbounded
+		oversized := strings.Repeat("a", 5*1024)
+		req := NewRequestWithBody(t, "PUT", fmt.Sprintf("%s/%s", tagsRoot, packageTag), strings.NewReader(oversized)).
+			AddTokenAuth(token)
+		MakeRequest(t, req, http.StatusRequestEntityTooLarge)
 	})
 
 	t.Run("ListTags", func(t *testing.T) {