feat(api): My Wave sessions with a Go fallback and wave metrics (M4)

- /wave/options, POST /wave/sessions (activity, mood, seed),
  /next with reactions, PATCH to switch the context on the fly, DELETE
- the recommender is called with an 800 ms budget; on failure the api
  serves a fallback wave of likes, favourite artists and random
  available tracks with the same dislike / recent / served filters
- every served track is an impression with its source and model; wave
  events (context_type wave) set its outcome, reactions win
- library status shows the analysis queue and 7-day wave metrics:
  listen-through, early skip and like rate

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

services/api/internal/wave/util.go

@@ -0,0 +1,16 @@
+package wave
+
+import (
+	"math/rand/v2"
+	"sort"
+)
+
+func shuffle[T any](in []T) []T {
+	out := append([]T(nil), in...)
+	rand.Shuffle(len(out), func(i, j int) { out[i], out[j] = out[j], out[i] })
+	return out
+}
+
+func stableSort[T any](s []T, less func(a, b T) bool) {
+	sort.SliceStable(s, func(i, j int) bool { return less(s[i], s[j]) })
+}