feat(server/playlists): manual Discover refresh endpoint

POST /api/playlists/system/discover/refresh re-runs the system
playlist build synchronously for the calling user, then returns
their newly-built Discover playlist's UUID and track count.

Used by the frontend's "Refresh" affordances (next task) on the
Discover detail page header and the home Playlists row tile kebab.
Operator's escape hatch when they want a different randomness
without waiting for tomorrow's cron tick (e.g., after pasting a
Last.fm key, after the daily cover-art enrichment expanded their
playable library, or just for the heck of it).

Authenticated user only; the authed sub-router's RequireUser
middleware handles 401. Each user refreshes only their own
Discover — no admin route, no cross-user impersonation.

Adds GetSystemPlaylistByVariantForUser sqlc query for the response
lookup. Returns playlist_id=null and track_count=0 if the build
succeeded but the user's library yielded no eligible tracks
(degenerate empty-library).

internal/db/queries/system_playlists.sql

@@ -154,3 +154,19 @@ INSERT INTO playlists (
     kind, system_variant, seed_artist_id, cover_path
 ) VALUES ($1, $2, '', false, 'system', $3, $4, $5)
 RETURNING *;
+
+-- name: GetSystemPlaylistByVariantForUser :one
+-- Returns the user's system playlist for the given variant
+-- ('for_you' / 'songs_like_artist' / 'discover'), or pgx.ErrNoRows
+-- when the user hasn't built that variant yet. For
+-- 'songs_like_artist' (which can have multiple rows per user, one
+-- per seed), this returns whichever LIMIT 1 picks — callers that
+-- need all instances should use a different query.
+SELECT p.id, p.user_id, p.name, p.kind, p.system_variant,
+       p.seed_artist_id, p.cover_path, p.track_count
+  FROM playlists p
+ WHERE p.user_id = $1
+   AND p.kind = 'system'
+   AND p.system_variant = $2
+ ORDER BY p.created_at DESC
+ LIMIT 1;