docs(api): sync mirrors with the retired rules CRUD and the chain-order endpoint

Three mirrors had drifted:

- The rules-endpoint retirement (4f71eda6) deleted the routes but not
  their mirrors: removed docs/developer/api/collections/
  {add_auto_assign_rule,remove_auto_assign_rule,test_rule,update_rule}
  .md + their bruno twins, and pruned /api/collections/test-rules,
  /{id}/rules, /{id}/rules/{ruleId} from the generated openapi spec
  (hand-edited — regenerating with -update would wipe hand-authored
  bruno suites that predate the docs mirrors).
- New endpoint PUT /api/collections/:id/view-settings gets a doc page
  + schema-registry entry + bruno mirror (repo convention: the route
  and its docs land together).
- 54 pre-existing TestDocsMatchRoutes failures: the media-item
  endpoints emit the 16 episode/download fields (source_feed_id,
  feed_position, download_state, enclosure_url, hosts/guests, …) and
  libraries emit purpose/allow_member_feeds, but the doc examples
  were never extended — the package only runs on the host, so the
  container suite never caught it. Examples now carry every key the
  handlers emit; internal/docs is fully green.

docs/developer/api/libraries/get_library_folders.md

@@ -18,7 +18,8 @@ Lists the filesystem folders configured for a library.
     "id": "aa0e8400-e29b-41d4-a716-446655440000",
     "library_id": "990e8400-e29b-41d4-a716-446655440000",
     "folder_path": "/books/manga",
-    "created_at": "2026-01-01T00:00:00Z"
+    "created_at": "2026-01-01T00:00:00Z",
+    "purpose": "scan"
   }
 ]
 ```