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/collections/set_chain_order.md

@@ -0,0 +1,48 @@
+---
+method: PUT
+path: /api/collections/:id/view-settings
+auth: jwt
+area: collections
+---
+<!-- source: internal/handlers/collections.go CollectionHandler.SetCollectionChainOrder -->
+
+# Set Collection Chain Order
+
+Sets the curated listening order for the collection's episode sequence
+(plan §10 R2): the chain producer (`buildSmartCollectionSequenceContext`)
+plays `Play as Sequence` in this order. The key merges into the
+collection's `view_settings` JSONB — playlist toggles that share the
+document survive the write. Owner-only.
+
+## Example request
+
+```json example-request
+{
+  "chain_order": "newest"
+}
+```
+
+## Example response
+
+```json example-response
+{
+  "id": "550e8400-e29b-41d4-a716-446655440000",
+  "chain_order": "newest",
+  "view_settings": { "chain_order": "newest" }
+}
+```
+
+## Fields
+
+| Field | Type | Required | Notes |
+| ----- | ---- | -------- | ----- |
+| chain_order | string | Yes | One of `native`, `newest`, `oldest`, `reverse`; `native`/empty removes the stored key |
+
+## Errors
+
+| Status | Body | When |
+| ------ | ---- | ---- |
+| 400 | `{"error": "invalid collection id"}` | malformed id |
+| 400 | `{"error": "chain_order must be one of: native, newest, oldest, reverse"}` | unknown order |
+| 404 | `{"error": "collection not found"}` | unknown id |
+| 401 / 403 | — | missing JWT / not the owner |