docs(swagger): rename notifications path placeholder user_id -> id (#838)

## Summary

Rename the OpenAPI path placeholder on the two notifications routes from
`{user_id}` to `{id}`, matching the convention used by every
`/users/{id}/...` route. Concrete URL is unchanged.

- `/notifications/{user_id}` → `/notifications/{id}`
- `/notifications/{user_id}/playlist_updates` →
`/notifications/{id}/playlist_updates`

## Why

Follow-up to #837, which added a `user_id` query parameter to both
routes for personalization of embedded `related.users`. With the path
placeholder *also* named `user_id`, `openapi-generator`
(typescript-fetch) emitted a colliding TS field (`userId2`) in the
generated `GetNotificationsRequest` / `GetPlaylistUpdatesRequest`. The
wire format was correct but the public SDK type surface was awkward.

Renaming the path placeholder to `id` lets the generator produce a clean
shape:

```ts
GetNotificationsRequest { id: string; userId?: string; ... }
```

The Fiber router matches `/notifications/:userId` by position, not by
name, so no Go change is required and the concrete URL clients send is
unchanged.

## Breaking change

This **is** a breaking change for external `@audius/sdk` consumers of
`getNotifications` / `getPlaylistUpdates`: the path field renames from
`userId` to `id`. Server behaviour, wire format, and URL paths are
unchanged.

In-monorepo call sites will be updated in the corresponding apps PR
([AudiusProject/apps#14366](AudiusProject/apps#14366))
after this merges.

## Test plan

- [x] Spec validates in CI.
- [x] Hitting `/notifications/<hashed-id>?user_id=<hashed-id>`
post-deploy returns the same response as before.
- [x] Regenerating the apps SDK against the merged spec produces a clean
`GetNotificationsRequest` with `id` (path) and `userId` (query), no
`userId2`.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

Author: raymondjacobson

api/swagger/swagger-v1.yaml

@@ -8181,14 +8181,14 @@ paths:
         "500":
           description: Server error
           content: {}
-  /notifications/{user_id}:
+  /notifications/{id}:
     get:
       tags:
         - notifications
       description: Get notifications for user ID
       operationId: Get Notifications
       parameters:
-        - name: user_id
+        - name: id
           in: path
           description: A User ID
           required: true
@@ -8199,8 +8199,8 @@ paths:
           description:
             The user ID of the user making the request. Required for
             personalization of related users (e.g. does_current_user_follow)
-            in the response, and may differ from the path user_id when a
-            manager is reading a managed user's notifications.
+            in the response, and may differ from the path id when a manager
+            is reading a managed user's notifications.
           schema:
             type: string
         - name: timestamp
@@ -8281,7 +8281,7 @@ paths:
             application/json:
               schema:
                 $ref: "#/components/schemas/notifications_response"
-  /notifications/{user_id}/playlist_updates:
+  /notifications/{id}/playlist_updates:
     get:
       tags:
         - notifications
@@ -8290,7 +8290,7 @@ paths:
         ID
       operationId: Get Playlist Updates
       parameters:
-        - name: user_id
+        - name: id
           in: path
           description: A User ID
           required: true
@@ -8301,8 +8301,8 @@ paths:
           description:
             The user ID of the user making the request. Required for
             personalization of related users (e.g. does_current_user_follow)
-            in the response, and may differ from the path user_id when a
-            manager is reading a managed user's playlist updates.
+            in the response, and may differ from the path id when a manager
+            is reading a managed user's playlist updates.
           schema:
             type: string
       responses: