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/main.py

@@ -81,6 +81,7 @@ class Config:
         'YTDL_OPTIONS_PRESETS': '{}',
         'YTDL_OPTIONS_PRESETS_FILE': '',
         'ALLOW_YTDL_OPTIONS_OVERRIDES': 'false',
+        'ALLOW_PRIVATE_ADDRESSES': 'false',
         'CORS_ALLOWED_ORIGINS': '',
         'ROBOTS_TXT': '',
         'HOST': '0.0.0.0',
@@ -96,7 +97,7 @@ class Config:
         'YTDL_NIGHTLY_UPDATE_TIME': '',
     }
 
-    _BOOLEAN = ('DOWNLOAD_DIRS_INDEXABLE', 'CUSTOM_DIRS', 'CREATE_CUSTOM_DIRS', 'DELETE_FILE_ON_TRASHCAN', 'HTTPS', 'ENABLE_ACCESSLOG', 'ALLOW_YTDL_OPTIONS_OVERRIDES')
+    _BOOLEAN = ('DOWNLOAD_DIRS_INDEXABLE', 'CUSTOM_DIRS', 'CREATE_CUSTOM_DIRS', 'DELETE_FILE_ON_TRASHCAN', 'HTTPS', 'ENABLE_ACCESSLOG', 'ALLOW_YTDL_OPTIONS_OVERRIDES', 'ALLOW_PRIVATE_ADDRESSES')
 
     def __init__(self):
         for k, v in self._DEFAULTS.items():