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_public_only_test.go

@@ -163,3 +163,22 @@ func assertPublicActivitiesOnly(t *testing.T, activities []api.Activity) {
 		}
 	}
 }
+
+// TestAPIRepoLimitedOwnerPublicOnly ensures a public-only token cannot reach a public repo
+// owned by a limited-visibility owner (which is not reachable anonymously).
+func TestAPIRepoLimitedOwnerPublicOnly(t *testing.T) {
+	defer tests.PrepareTestEnv(t)()
+
+	// limited_org (limited visibility) owns the non-private repo public_repo_on_limited_org
+	const url = "/api/v1/repos/limited_org/public_repo_on_limited_org"
+
+	// a public-only token is confined to genuinely public resources
+	publicOnlyToken := getUserToken(t, "user2", auth_model.AccessTokenScopeReadRepository, auth_model.AccessTokenScopePublicOnly)
+	req := NewRequest(t, "GET", url).AddTokenAuth(publicOnlyToken)
+	MakeRequest(t, req, http.StatusNotFound)
+
+	// a normal token can still reach the limited owner's public repo
+	token := getUserToken(t, "user2", auth_model.AccessTokenScopeReadRepository)
+	req = NewRequest(t, "GET", url).AddTokenAuth(token)
+	MakeRequest(t, req, http.StatusOK)
+}