Provider layer + hianime source (primary)

- providers/{anidb,anikoto,hianime}.py: per-source modules behind the
  ANI_SOURCE fallback dispatch (upstream.py _call/_providers)
- hianime.py: pystardust/ani-cli#1897 flow — search/episode-list/
  servers API, HD-1 hash -> base64 embed url, window.__P blob
  (base64 + XOR 'otaku-embed-v1') -> master m3u8; registers CDN hosts
  and the required Referer via stream_meta()
- upstream.py: get_stream/fetch_parallel accept extra headers;
  default source order hianime,anidb,anikoto (anidb.app is 503)
- app.py: /stream allowlist accepts provider-registered hosts and
  sends their Referer; status probe follows the primary provider
  (hianime variant URIs are relative to the variant path)
- app.js: error-time position save floors at furthest point reached
  (hls.js resets currentTime to 0 before playerFail on fatal errors)
- tests: default-order + hianime import checks; live browser suite green

server/providers/__init__.py

@@ -0,0 +1 @@
+"""providers/ — anime source providers. See upstream.py for the interface."""