feat: add edit artist endpoint (section 1.4)

- Add GET/PUT /api/artists/{id} for artist settings
- Update sync to create artists table entry (library settings)
- Support partial updates for monitored, path, quality/metadata profiles
- Add e2e tests for get, edit, partial update flows

internal/api/handlers.go

@@ -323,6 +323,54 @@ func (h *Handlers) DeleteArtist(w http.ResponseWriter, r *http.Request) {
 	})
 }
 
+func (h *Handlers) GetArtist(w http.ResponseWriter, r *http.Request) {
+	if h.DB == nil {
+		writeError(w, http.StatusServiceUnavailable, "database not connected")
+		return
+	}
+
+	artistID := chi.URLParam(r, "id")
+	if artistID == "" {
+		writeError(w, http.StatusBadRequest, "artist ID required")
+		return
+	}
+
+	artist, err := h.DB.GetArtistByForeignID(r.Context(), artistID)
+	if err != nil {
+		writeError(w, http.StatusNotFound, "artist not found: "+artistID)
+		return
+	}
+
+	writeJSON(w, http.StatusOK, artist)
+}
+
+func (h *Handlers) EditArtist(w http.ResponseWriter, r *http.Request) {
+	if h.DB == nil {
+		writeError(w, http.StatusServiceUnavailable, "database not connected")
+		return
+	}
+
+	artistID := chi.URLParam(r, "id")
+	if artistID == "" {
+		writeError(w, http.StatusBadRequest, "artist ID required")
+		return
+	}
+
+	var update database.ArtistUpdate
+	if err := json.NewDecoder(r.Body).Decode(&update); err != nil {
+		writeError(w, http.StatusBadRequest, "invalid request body")
+		return
+	}
+
+	artist, err := h.DB.UpdateArtistByForeignID(r.Context(), artistID, update)
+	if err != nil {
+		writeError(w, http.StatusNotFound, "artist not found: "+artistID)
+		return
+	}
+
+	writeJSON(w, http.StatusOK, artist)
+}
+
 func writeJSON(w http.ResponseWriter, status int, v any) {
 	w.Header().Set("Content-Type", "application/json")
 	w.WriteHeader(status)