feat(release): distinguish streaming distributors from sources

Introduce a separate dimension for streaming-platform tags (NF, AMZN,
DSNP, HMAX, ATVP, …) so they stop polluting the encoding-source field.
WEB-DL is the source; the platform that released it is the distributor.

- new distributors.yaml knowledge file
- ReleaseKnowledge port exposes distributors set
- TokenRole.DISTRIBUTOR + ParsedRelease.distributor field
- removed NF/AMZN/DSNP/HMAX/ATVP from sources.yaml
- notre_planete fixture now records distributor: NF

tests/fixtures/releases/shitty/notre_planete_lowercase_e/expected.yaml

@@ -1,7 +1,8 @@
 release_name: "Notre.planete.s01e01.1080p.NF.WEB-DL.DDP5.1.x264-NTb"
 
 # Lowercase 's01e01' and lowercased title word ('planete') correctly parsed.
-# NF (Netflix) source tag is not in the source KB — drops; WEB-DL wins.
+# NF is the Netflix streaming distributor (separate dimension from source);
+# WEB-DL is the encoding source.
 parsed:
   title: "Notre.planete"
   year: null
@@ -11,6 +12,7 @@ parsed:
   source: "WEB-DL"
   codec: "x264"
   group: "NTb"
+  distributor: "NF"
   tech_string: "1080p.WEB-DL.x264"
   media_type: "tv_show"
   parse_path: "direct"