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/cmd/api/main.go

@@ -6,6 +6,7 @@ import (
 	"crypto/ed25519"
 	"fmt"
 	"log/slog"
+	"net/http"
 	"os"
 	"os/signal"
 	"path/filepath"
@@ -28,6 +29,7 @@ import (
 	"pipu/api/internal/search"
 	"pipu/api/internal/server"
 	"pipu/api/internal/system"
+	"pipu/api/internal/wave"
 )
 
 func main() {
@@ -140,6 +142,8 @@ func run() error {
 		Library:     library.NewHandler(libSvc, libStore, dto, covers, cfg.XAccel),
 		Search:      search.NewHandler(meili, libStore, indexer, dto, log),
 		Playback:    pb,
+		Wave: wave.NewHandler(pool, &wave.Recommender{URL: cfg.RecommenderURL, Token: cfg.InternalToken,
+			Client: &http.Client{}}, libStore, func() library.DTO { return dto }, log),
 	}))
 	log.Info("starting", "version", buildinfo.Version)
 	return httpx.Serve(ctx, log, srv, cfg.ShutdownTimeout)