[arena] add 'channel' extractor (#5847 #8509)

* feat: issue 5847 site support for are.na
* flake8 lint error fix
* class name error fix
* update
    - prevent unnecessary request to 'page_url'
    - fix pagination
    - simplify block extraction code
    TODO: - rewrite without GalleryExtractor
          - extractors for Blocks, Users, etc
* supportedsites
* tests
* rename to 'channel' extractor
* update site title to 'Are.na'
* prioritize attachments

---------

Co-authored-by: Mike Fährmann <mike_faehrmann@web.de>

gallery_dl/extractor/__init__.py

@@ -26,6 +26,7 @@ modules = [
     "ao3",
     "arcalive",
     "architizer",
+    "arena",
     "artstation",
     "aryion",
     "batoto",

gallery_dl/extractor/arena.py

@@ -0,0 +1,85 @@
+# -*- coding: utf-8 -*-
+
+# Copyright 2025 Mike Fährmann
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License version 2 as
+# published by the Free Software Foundation.
+
+"""Extractor for https://are.na/"""
+
+from .common import GalleryExtractor
+
+
+class ArenaChannelExtractor(GalleryExtractor):
+    """Extractor for are.na channels"""
+    category = "arena"
+    subcategory = "channel"
+    root = "https://are.na"
+    pattern = r"(?:https?://)?(?:www\.)?are\.na/[^/?#]+/([^/?#]+)"
+    example = "https://are.na/evan-collins-1522646491/cassette-futurism"
+
+    def metadata(self, page):
+        info = self.request_json(
+            f"https://api.are.na/v2/channels/{self.groups[0]}")
+
+        return {
+            "gallery_id"  : info.get("slug") or str(info.get("id")),
+            "channel_id"  : info.get("id"),
+            "channel_slug": info.get("slug"),
+            "title"       : info.get("title") or "",
+            "count"       : info.get("length") or 0,
+            "user"        : info.get("user"),
+            "date"        : self.parse_datetime_iso(info.get("created_at")),
+            "date_updated": self.parse_datetime_iso(info.get("updated_at")),
+        }
+
+    def images(self, page):
+        api = f"https://api.are.na/v2/channels/{self.groups[0]}/contents"
+        limit = 100
+        params = {"page": 1, "per": limit}
+
+        while True:
+            data = self.request_json(api, params=params)
+
+            contents = data.get("contents")
+            if not contents:
+                return
+
+            for block in contents:
+                url = None
+                meta = {
+                    "id": block.get("id"),
+                    "block_class": block.get("class"),
+                    "block_title": block.get("title") or block.get(
+                        "generated_title") or "",
+                }
+
+                # Attachments (e.g., PDFs, files)
+                if attachment := block.get("attachment"):
+                    url = attachment.get("url")
+
+                # Images
+                elif image := block.get("image"):
+                    # Prefer original image
+                    if original := image.get("original"):
+                        url = original.get("url")
+                    # Fallback to display/large image if present
+                    elif display := image.get("display"):
+                        url = display.get("url")
+                    elif large := image.get("large"):
+                        url = large.get("url")
+
+                # Some Links/Channels may not have downloadable media
+                if not url:
+                    continue
+
+                # Provide source link if it exists
+                if src := block.get("source"):
+                    meta["source_url"] = src.get("url") or ""
+
+                yield url, meta
+
+            if len(contents) < limit:
+                return
+            params["page"] += 1