[Feed] Add GET /v1/users/{id}/feed/for-you endpoint

Server-side replacement for the client-side blend in apps'
useForYouFeed.ts. Four candidate streams interleaved with a fixed
10-slot pattern,

  pos: [R R R F R T R F U R]

giving 60% recommended (50% baseline + 10% filler when other sources
thin out), 20% following originals, 10% trending, 10% underground.
When a slot's preferred source is exhausted, falls through in priority
order recommended → following → trending → underground.

Each source caps at 200 candidates; the union is deduped by track_id
and the caller's already-saved tracks are filtered at the SQL level
(client filtered them client-side; filtering early avoids burning
candidates on already-saved tracks). Pagination (limit/offset)
applies to the composed list.

Source SQL mirrors the underlying single-source endpoints:
- recommended: v1_users_recommended_tracks.go (top tracks from the
  user's top-played genres, excluding played) — switched to
  score-DESC ordering for stable pagination
- following:   v1_users_feed.go w/ filter=original
- trending:    v1_tracks_trending.go (week)
- underground: v1_tracks_trending_underground.go (week, sub-1500
  follower & following artists)

Liveness, gating, and unlisted/deleted filters mirror those files
and v1_events_remix_contests.go.

Path-param userId (the requesting user) follows the same pattern as
/v1/users/:userId/recommended-tracks and /v1/users/:userId/feed.

Consumed by apps' useForYouFeed.ts.

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

Author: dylanjeffers

api/server.go

@@ -426,6 +426,7 @@ func NewApiServer(config config.Config) *ApiServer {
 		g.Get("/users/:userId/albums", app.v1UserAlbums)
 		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

@@ -9439,6 +9439,66 @@ 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 track feed for the user. Mirrors
+        the client-side blend of recommended, following originals, weekly
+        trending, and underground trending using a fixed 10-slot
+        interleave (60% recommended, 20% following, 10% trending, 10%
+        underground). Already-saved tracks are filtered out and results
+        are deduped by track ID.
+      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: 10
+            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: 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: