chore: remove /v1/users/{id}/feed/for-you endpoint (#807)

## Summary

Retiring the dedicated For You feed endpoint. The clients are being
switched to use \`/v1/users/{id}/recommended-tracks\` instead — the same
endpoint that already powers the Explore page's For You section and
works fine in production. See companion PR: AudiusProject/apps#14301.

## Why

The custom \`/feed/for-you\` endpoint had repeated issues since it
shipped:

* **Auth gate bug** (fixed in #804) — global authMiddleware rejected
unsigned \`user_id\` requests, making the endpoint unreachable from the
web RC.
* **Perf** — even after #805 and #806 capped the \`my_saved_artists\`,
\`my_artist_affinity\`, and \`follow_set\` CTEs, EXPLAIN on prod showed
the \`similar_artists\` self-join still produced a 301M-row merge for
power users (and a fixed ~12s \`track_trending_scores\` scan for *every*
user due to a missing partial index). The endpoint never reliably
completed within Cloudflare's 100s upstream limit for power users.
* **Duplication** — the response shape (ranked track list for the
signed-in user) is already what \`/recommended-tracks\` returns. Two
endpoints solving the same problem isn't worth maintaining.

Consolidating on the working endpoint is simpler than continuing to
optimize the custom one.

## Removed

| File | What |
|---|---|
| \`api/v1_users_feed_for_you.go\` | Handler + the 200-row
candidate-pool SQL (4 candidate sources, similar_artists CF, diversity
pass) |
| \`api/v1_users_feed_for_you_test.go\` | 9 unit tests |
| \`api/server.go\` (1 line) | Route registration |
| \`api/auth_middleware.go\` (~10 lines) | The \`/feed/for-you\`
exemption from #804 — no longer needed |
| \`api/swagger/swagger-v1.yaml\` (~70 lines) | The endpoint's swagger
entry |

## Test plan

- ✅ \`go build ./api/...\` clean
- ✅ \`go vet ./api/...\` clean
- ✅ All remaining \`TestV1UsersFeed*\` / \`TestAuth*\` tests pass
locally
- After merge + deploy + AudiusProject/apps#14301 deploy: Feed → For You
tab on the web RC should show the same recommended tracks as Explore's
For You section.

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

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: dylanjeffers

api/auth_middleware.go

@@ -347,16 +347,8 @@ func (app *ApiServer) authMiddleware(c *fiber.Ctx) error {
 		return fiber.NewError(fiber.StatusUnauthorized, "Invalid or expired access token")
 	}
 
-	// Not authorized to act on behalf of myId.
-	//
-	// Exception: /users/:userId/feed/for-you accepts user_id as a viewer hint
-	// used only for response decoration (has_current_user_reposted etc.); the
-	// path :userId — not user_id — controls what gets personalized. Treat the
-	// query user_id as advisory rather than authoritative on this route so
-	// the endpoint can be called like the other public read endpoints.
-	allowUnauthenticatedViewerId := strings.HasSuffix(c.Path(), "/feed/for-you")
-
-	if myId != 0 && !pkceAuthed && !allowUnauthenticatedViewerId && !app.isAuthorizedRequest(c.Context(), myId, wallet) {
+	// Not authorized to act on behalf of myId
+	if myId != 0 && !pkceAuthed && !app.isAuthorizedRequest(c.Context(), myId, wallet) {
 		return fiber.NewError(
 			fiber.StatusForbidden,
 			fmt.Sprintf(

api/server.go

@@ -453,7 +453,6 @@ func NewApiServer(config config.Config) *ApiServer {
 		g.Get("/users/:userId/contests", app.v1UserContests)
 		g.Get("/users/:userId/playlists", app.v1UserPlaylists)
 		g.Get("/users/:userId/feed", app.v1UsersFeed)
-		g.Get("/users/:userId/feed/for-you", app.v1UsersFeedForYou)
 		g.Get("/users/:userId/connected_wallets", app.v1UsersConnectedWallets)
 		g.Get("/users/:userId/transactions/audio", app.v1UsersTransactionsAudio)
 		g.Get("/users/:userId/transactions/audio/count", app.v1UsersTransactionsAudioCount)

api/swagger/swagger-v1.yaml

@@ -9433,76 +9433,6 @@ paths:
         "500":
           description: Server error
           content: {}
-  /users/{id}/feed/for-you:
-    get:
-      tags:
-        - users
-      summary: Get For You feed for user
-      description:
-        Returns a personalized For You feed for the user identified in the
-        path. Twitter-style multi-source pipeline — candidate retrieval
-        (in-network, trending, underground, similar-artist) → linear
-        ranking (recency decay × engagement × social affinity, weighted
-        by source) → diversity (per-artist cap + consecutive-same-artist
-        lookahead).
-      operationId: Get User For You Feed
-      security:
-        - {}
-        - OAuth2:
-            - read
-      parameters:
-        - name: id
-          in: path
-          description: A User ID
-          required: true
-          schema:
-            type: string
-        - name: limit
-          in: query
-          description: The number of items to fetch
-          schema:
-            type: integer
-            default: 25
-            minimum: 1
-            maximum: 100
-        - name: offset
-          in: query
-          description:
-            The number of items to skip. Useful for pagination (page number
-            * limit)
-          schema:
-            type: integer
-            default: 0
-            minimum: 0
-            maximum: 200
-        - name: max_per_artist
-          in: query
-          description:
-            Maximum number of tracks per artist on a single page. Used by the
-            diversity pass to cap consecutive same-artist results.
-          schema:
-            type: integer
-            default: 3
-            minimum: 1
-            maximum: 10
-        - name: user_id
-          in: query
-          description: The user ID of the user making the request
-          schema:
-            type: string
-      responses:
-        "200":
-          description: Success
-          content:
-            application/json:
-              schema:
-                $ref: "#/components/schemas/tracks"
-        "400":
-          description: Bad request
-          content: {}
-        "500":
-          description: Server error
-          content: {}
   /users/{id}/library/albums:
     get:
       tags: