feat(api): GET /api/albums/{id}/cover with sidecar fallback

internal/api/media.go

@@ -8,10 +8,15 @@ package api
 
 import (
 	"context"
+	"errors"
+	"net/http"
 	"os"
 	"path/filepath"
 	"strings"
 
+	"github.com/go-chi/chi/v5"
+	"github.com/jackc/pgx/v5"
+
 	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
 )
 
@@ -96,3 +101,49 @@ func imageContentType(path string) string {
 	}
 	return "application/octet-stream"
 }
+
+// handleGetCover implements GET /api/albums/{id}/cover. Resolves the cover
+// path (explicit column, else sidecar next to first track), then delegates
+// byte-serving to http.ServeContent so clients get Range / If-Modified-Since
+// / ETag for free.
+func (h *handlers) handleGetCover(w http.ResponseWriter, r *http.Request) {
+	id, ok := parseUUID(chi.URLParam(r, "id"))
+	if !ok {
+		writeErr(w, http.StatusBadRequest, "bad_request", "invalid album id")
+		return
+	}
+	q := dbq.New(h.pool)
+	album, err := q.GetAlbumByID(r.Context(), id)
+	if err != nil {
+		if errors.Is(err, pgx.ErrNoRows) {
+			writeErr(w, http.StatusNotFound, "not_found", "album not found")
+			return
+		}
+		h.logger.Error("api: get album for cover failed", "err", err)
+		writeErr(w, http.StatusInternalServerError, "server_error", "server error")
+		return
+	}
+	path := resolveAlbumCoverPath(r.Context(), q, album)
+	if path == "" {
+		writeErr(w, http.StatusNotFound, "not_found", "cover not found")
+		return
+	}
+	f, err := os.Open(path)
+	if err != nil {
+		// resolveAlbumCoverPath saw the file a moment ago; if it vanished
+		// between stat and open, treat it the same as no-art — the user
+		// experience (404) matches, and 500 would misleadingly imply a server
+		// bug rather than a filesystem race.
+		writeErr(w, http.StatusNotFound, "not_found", "cover not found")
+		return
+	}
+	defer func() { _ = f.Close() }()
+	info, err := f.Stat()
+	if err != nil {
+		h.logger.Error("api: stat cover failed", "err", err)
+		writeErr(w, http.StatusInternalServerError, "server_error", "server error")
+		return
+	}
+	w.Header().Set("Content-Type", imageContentType(path))
+	http.ServeContent(w, r, filepath.Base(path), info.ModTime(), f)
+}