feat: ALLOW_PRIVATE_ADDRESSES to opt out of the SSRF checks (closes #1036)

The SSRF guard rejects any address that isn't globally routable, which breaks
proxy/VPN setups that resolve hosts into private or special-use ranges. The
reported case is Fake-IP clients (sing-box, Clash, Mihomo) that map YouTube to
the RFC 2544 benchmarking range 198.18.0.0/15 to tunnel the traffic; MeTube
rejected it with "Refusing to fetch internal address" even though yt-dlp itself
handles it fine.

Add a boolean ALLOW_PRIVATE_ADDRESSES (default false) that, when set, skips both
layers: validate_url returns after scheme validation without the internal-host
checks, and the connect-time socket guard is not installed. Threaded to the
download subprocess via Download so the guard sees it too. Scheme validation
(http/https only) still applies. Documented in the README as a trust-your-network
opt-out that disables SSRF protection.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

app/tests/test_url_guard.py

@@ -145,6 +145,32 @@ class GuardedGetaddrinfoTests(unittest.TestCase):
         self.assertEqual([r[4][0] for r in results], ["127.0.0.1"])
 
 
+class AllowPrivateBypassTests(unittest.TestCase):
+    """ALLOW_PRIVATE_ADDRESSES: trusted proxy/VPN environments opt out of the
+    SSRF address checks (e.g. Fake-IP clients that resolve to 198.18.0.0/15)."""
+
+    def test_internal_address_allowed_when_bypassed(self):
+        # Fake-IP benchmarking range that is_global rejects by default.
+        self.assertIsNone(validate_url("http://www.youtube.com/x", allow_private=True))
+
+    def test_private_host_allowed_when_bypassed(self):
+        # No DNS lookup needed: the bypass returns before resolution.
+        with mock.patch("url_guard.socket.getaddrinfo") as gai:
+            self.assertIsNone(validate_url("http://192.168.1.1/x", allow_private=True))
+            gai.assert_not_called()
+
+    def test_scheme_still_enforced_when_bypassed(self):
+        self.assertIsNotNone(validate_url("file:///etc/passwd", allow_private=True))
+
+    def test_socket_guard_not_installed_when_bypassed(self):
+        original = socket.getaddrinfo
+        try:
+            install_socket_guard(allow_private=True)
+            self.assertIs(socket.getaddrinfo, original)
+        finally:
+            socket.getaddrinfo = original
+
+
 class InstallSocketGuardTests(unittest.TestCase):
     def test_install_replaces_and_is_idempotent(self):
         original = socket.getaddrinfo