feat(db): add LoadRadioCandidates query

Single SELECT that joins tracks with general_likes (for is_liked) and
an aggregated LATERAL subquery on play_events (for last_played_at,
play_count, skip_count). Excludes seed + tracks played in the last
N hours. Drives the M3 weighted shuffle scoring.

internal/db/queries/recommendation.sql

@@ -0,0 +1,29 @@
+-- name: LoadRadioCandidates :many
+-- Returns all tracks except the seed and any played by the user within
+-- the last $3 hours, joined with the stats needed for scoring:
+--   is_liked     — boolean from general_likes for this user
+--   last_played_at — max(play_events.started_at) for this user/track
+--   play_count   — count of play_events for this user/track
+--   skip_count   — play_events where was_skipped=true
+SELECT
+    sqlc.embed(t),
+    (l.user_id IS NOT NULL)::bool AS is_liked,
+    pe.last_played_at::timestamptz AS last_played_at,
+    pe.play_count,
+    pe.skip_count
+FROM tracks t
+LEFT JOIN general_likes l ON l.user_id = $1 AND l.track_id = t.id
+LEFT JOIN LATERAL (
+    SELECT
+        max(started_at) AS last_played_at,
+        count(*) AS play_count,
+        count(*) FILTER (WHERE was_skipped) AS skip_count
+    FROM play_events
+    WHERE user_id = $1 AND track_id = t.id
+) pe ON true
+WHERE t.id <> $2
+  AND NOT EXISTS (
+    SELECT 1 FROM play_events
+    WHERE user_id = $1 AND track_id = t.id
+      AND started_at > now() - $3 * interval '1 hour'
+  );