feat(arr): add opensubtitles.com subtitle provider

Searches by moviehash computed from the media file and by TMDB id
with season/episode for TV, ranks candidates through
arr_core::subs::rank, and downloads under a lazily-fetched user
token. The daily download cap (429/406) surfaces as Error::RateLimited
so the loop can show a queue state. Credentials come from config or
environment only; tests run against wiremock fixtures.

crates/arr-subs/tests/fixtures/subtitles_search_episode.json

@@ -0,0 +1,23 @@
+{
+  "total_count": "1",
+  "data": [
+    {
+      "id": "7111111",
+      "type": "subtitle",
+      "attributes": {
+        "subtitle_id": "7111111",
+        "language": "pt-PT",
+        "release": "Series.S01E02.1080p.WEB-DL-GRP",
+        "moviehash_match": false,
+        "ratings": 60.0,
+        "download_count": 77,
+        "hearing_impaired": false,
+        "foreign_parts_only": false,
+        "format": "ass",
+        "files": [
+          { "file_id": 61111111, "file_name": "series.s01e02.pt.ass", "cd_number": 1 }
+        ]
+      }
+    }
+  ]
+}