Add the step-by-step fixer and library types

Each pair is a Music (Artist/Album/01 Title) or Soundtracks (Album/01 Title)
library. The new Fixer tab takes the master, then the slave, through four
reviewed steps: folder structure, album tags (optional MusicBrainz lookup;
the slave reuses the fixed master's tags), folder names and file names,
then links the two libraries. Every fix is an undoable operation.

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

internal/api/fixer.go

@@ -0,0 +1,94 @@
+package api
+
+import (
+	"net/http"
+
+	"github.com/cdnutter/playlink/internal/engine"
+)
+
+// ---- step-by-step fixer ----
+
+func (s *Server) handleFixer(w http.ResponseWriter, r *http.Request) {
+	id, _ := idParam(r)
+	p, err := s.st.Pair(r.Context(), id)
+	if err != nil {
+		opErr(w, err)
+		return
+	}
+	out := map[string]any{"state": engine.FixerStateOf(p), "steps": engine.FixerSteps, "plan": nil,
+		"items": []engine.FixItem{}, "albums": []*engine.AlbumFix{}}
+	if plan, items, albums := s.eng.FixerPlan(id); plan != nil {
+		out["plan"], out["items"], out["albums"] = plan, items, albums
+	}
+	writeJSON(w, 200, out)
+}
+
+func (s *Server) handleFixCheck(w http.ResponseWriter, r *http.Request) {
+	id, _ := idParam(r)
+	var body engine.FixCheck
+	if !readJSON(w, r, &body) {
+		return
+	}
+	j, err := s.eng.StartFixCheck(id, body)
+	if err != nil {
+		opErr(w, err)
+		return
+	}
+	writeJSON(w, 202, j)
+}
+
+func (s *Server) handleFixApply(w http.ResponseWriter, r *http.Request) {
+	id, _ := idParam(r)
+	var body engine.FixApply
+	if !readJSON(w, r, &body) {
+		return
+	}
+	if !s.preAction(w, r, id, "fixer") {
+		return
+	}
+	j, err := s.eng.StartFixApply(id, body)
+	if err != nil {
+		opErr(w, err)
+		return
+	}
+	s.log.Info("fixer apply started", "pair", id, "selected", len(body.IDs))
+	writeJSON(w, 202, j)
+}
+
+func (s *Server) handleFixStep(w http.ResponseWriter, r *http.Request) {
+	id, _ := idParam(r)
+	var body struct {
+		Side  string `json:"side"` // master, slave, or "" for the link step
+		Step  string `json:"step"`
+		State string `json:"state"` // done, skipped or "" (to do)
+	}
+	if !readJSON(w, r, &body) {
+		return
+	}
+	if err := s.eng.SetStep(r.Context(), id, body.Side, body.Step, body.State, ""); err != nil {
+		opErr(w, err)
+		return
+	}
+	s.handleFixer(w, r)
+}
+
+func (s *Server) handleFixRelease(w http.ResponseWriter, r *http.Request) {
+	id, _ := idParam(r)
+	var body struct {
+		Album   int    `json:"album"`
+		Release string `json:"release"` // a MusicBrainz release id, or "search"
+	}
+	if !readJSON(w, r, &body) {
+		return
+	}
+	if j := s.eng.Job(id); j != nil && j.State == "running" {
+		writeErr(w, 409, engine.ErrBusy)
+		return
+	}
+	a, err := s.eng.ChooseRelease(r.Context(), id, body.Album, body.Release)
+	if err != nil {
+		opErr(w, err)
+		return
+	}
+	writeJSON(w, 200, a)
+}