fix: accept HOST=* so IPv6 users get a dual-stack bind (closes #795)

aiohttp hands HOST straight to getaddrinfo, which has no notion of a '*'
wildcard: the lookup fails and MeTube dies at startup on an opaque DNS
error. '*' is nevertheless what people reach for when they want to serve
both IP stacks -- it is the answer given on #795 -- while the value that
actually does it, an empty string, is undiscoverable.

asyncio expands an empty host to one listening socket per address family,
so map '*' onto it. Verified against the real stack:

    ''        -> [('0.0.0.0', p), ('::', p, 0, 0)]
    '0.0.0.0' -> [('0.0.0.0', p)]
    '::'      -> [('::', p, 0, 0)]
    '*'       -> gaierror (before this change)

The README claimed the 0.0.0.0 default was "all interfaces", which is
only true of IPv4; document the three modes instead. Note that '::' is
IPv6-only whatever the host's bindv6only says, because asyncio always
sets IPV6_V6ONLY on the sockets it binds.

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

app/main.py

@@ -113,6 +113,18 @@ class Config:
                     sys.exit(1)
                 setattr(self, k, v in ('true', 'True', 'on', '1'))
 
+        # aiohttp hands HOST straight to getaddrinfo, which has no notion of a
+        # '*' wildcard: the lookup fails and takes the server down at startup
+        # with an opaque DNS error. '*' is nevertheless what people reach for
+        # when they want to serve both IP stacks, while the value that actually
+        # does it -- an empty string, which asyncio expands to one listening
+        # socket per address family -- is undiscoverable. Accept '*' as the
+        # spelling for "every interface, both stacks". Note that '::' on its own
+        # is IPv6-only regardless of the host's bindv6only setting, because
+        # asyncio always sets IPV6_V6ONLY on the sockets it binds.
+        if self.HOST.strip() == '*':
+            self.HOST = ''
+
         if not self.URL_PREFIX.endswith('/'):
             self.URL_PREFIX += '/'
 
@@ -1386,7 +1398,9 @@ def isAccessLogEnabled():
 
 if __name__ == '__main__':
     logging.getLogger().setLevel(parseLogLevel(config.LOGLEVEL) or logging.INFO)
-    log.info(f"Listening on {config.HOST}:{config.PORT}")
+    # An empty HOST binds every interface on both stacks; print the '*' spelling
+    # that selects it rather than a bare ':8081'.
+    log.info(f"Listening on {config.HOST or '*'}:{config.PORT}")
 
 
     # Auto-detect cookie file on startup