feat(api): GET /api/artists with alpha/newest sort and paging

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/api/convert.go

@@ -1,11 +1,18 @@
 package api
 
 import (
+	"errors"
+	"net/url"
+	"strconv"
+	"strings"
+
 	"github.com/jackc/pgx/v5/pgtype"
 
 	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
 )
 
+var errBadPaging = errors.New("invalid limit or offset")
+
 // uuidToString renders a pgtype.UUID as a canonical hyphenated string.
 // Returns "" when the UUID is invalid (unset). Duplicated from the subsonic
 // package intentionally — the two packages must not depend on each other.
@@ -115,3 +122,38 @@ func trackRefFrom(t dbq.Track, albumTitle, artistName string) TrackRef {
 	}
 	return ref
 }
+
+// parsePaging reads limit/offset from the query string, applying defaults
+// and clamping. Returns a 400-worthy error when the values are non-numeric
+// or negative; out-of-range values silently clamp (deliberate — UX).
+func parsePaging(raw url.Values) (limit, offset int, err error) {
+	const (
+		defLimit = 50
+		maxLimit = 200
+	)
+	limit = defLimit
+	if s := strings.TrimSpace(raw.Get("limit")); s != "" {
+		n, perr := strconv.Atoi(s)
+		if perr != nil {
+			return 0, 0, errBadPaging
+		}
+		if n < 1 {
+			n = 1
+		}
+		if n > maxLimit {
+			n = maxLimit
+		}
+		limit = n
+	}
+	if s := strings.TrimSpace(raw.Get("offset")); s != "" {
+		n, perr := strconv.Atoi(s)
+		if perr != nil {
+			return 0, 0, errBadPaging
+		}
+		if n < 0 {
+			return 0, 0, errBadPaging
+		}
+		offset = n
+	}
+	return limit, offset, nil
+}