Keep hianime alive across server rotation: try every listed server

hianime's HD-1 slot rotated to a megaplay chain whose CDN (norami.top)
is Cloudflare-blocked for us, which killed stream resolution (CDN probe
sat at 0 KB/s). The ZokoAnime server still serves the working
zokoanime chain. _master_m3u8 now walks every listed server of the
wanted type; _embed_payload handles both chains (window.__P blob and
megaplay data-id/getSourcesNew with re-roll). stream_meta returns {}
when the host is ours but no referer is needed, so the /stream
allowlist still holds.

server/upstream.py

@@ -196,12 +196,15 @@ master_m3u8 = lambda e, l="eng": _call("master_m3u8", e, l)
 
 def stream_meta(url):
     """Provider hook: extra upstream headers for a /stream URL (hianime's
-    CDN demands the embed origin as Referer), or None if no enabled
-    provider claims the host — that doubles as the allowlist."""
+    CDN may demand the embed origin as Referer), or None if no enabled
+    provider claims the host — that doubles as the allowlist (an empty
+    dict means: allowed, no extra headers)."""
     for p in _providers():
         fn = getattr(_prov(p), "stream_meta", None)
-        if fn and fn(url):
-            return fn(url)
+        if fn:
+            v = fn(url)
+            if v is not None:
+                return v
     return None