Dispatch handleError via HTTPStatuser interface (4a)

handleError previously hard-coded a switch mapping every service sentinel to
its HTTP status. Adding a new sentinel required editing handleError (OCP
violation). Move the status onto the sentinel itself: introduce
api.HTTPStatuser { HTTPStatus() int }, convert the 10 dispatched service
sentinels to a typed *apiError that implements it, and turn handleError into
a thin errors.As-based dispatcher. ErrShareExpired stays a plain sentinel
because it is handled inline by share handlers and never reaches
handleError.

Sentinel messages are aligned with what handleError used to emit (e.g.
ErrForbidden is now "forbidden" instead of "access denied",
ErrAlreadyBootstrapped is "bootstrap already complete") so the dispatcher
can use err.Error() uniformly. Wrapped errors (fmt.Errorf("%w: ...",
ErrNotFound, ...)) keep their context in the response body, which is a
minor improvement over the previous behaviour that collapsed everything to
the bare sentinel message. errors.Is keeps working via pointer equality on
the *apiError sentinels.

1 files changed, 42 insertions, 12 deletions

diff --git a/player-server/internal/service/service.go b/player-server/internal/service/service.go
index b4ae127..beeb5aa 100644
--- a/player-server/internal/service/service.go
+++ b/player-server/internal/service/service.go
@@ -5,24 +5,54 @@ import (
 	"context"
 	"errors"
 	"io"
+	"net/http"
 	"time"
 
 	"codeberg.org/snonux/player/internal/model"
 )
 
-// Sentinel errors returned by the service layer.
+// apiError is a sentinel error that knows its own HTTP status. It implements
+// the api.HTTPStatuser interface (which is satisfied structurally — no import
+// of the api package is needed here) so that api.handleError can dispatch
+// based on the sentinel's own metadata instead of an ever-growing switch.
+//
+// Sentinels are declared as *apiError pointers, which makes errors.Is work via
+// pointer equality even when the error is wrapped with fmt.Errorf("%w: …").
+type apiError struct {
+	msg    string
+	status int
+}
+
+// Error returns the sentinel's human-readable message.
+func (e *apiError) Error() string { return e.msg }
+
+// HTTPStatus returns the HTTP status code this sentinel should map to.
+func (e *apiError) HTTPStatus() int { return e.status }
+
+// Sentinel errors returned by the service layer. Each one carries the HTTP
+// status that the API layer should emit, so handleError stays open for
+// extension (new sentinel) but closed for modification.
+//
+// Where the previous handleError emitted a fixed message that differed from
+// the sentinel's text (e.g. "forbidden" instead of "access denied"), the
+// sentinel text now matches that message so the dispatcher can use the
+// error chain's own text uniformly.
 var (
-	ErrNotFound             = errors.New("not found")
-	ErrForbidden            = errors.New("access denied")
-	ErrShareNotFound        = errors.New("share not found")
-	ErrShareExpired         = errors.New("share expired")
-	ErrMediaNotFound        = errors.New("media not found")
-	ErrUnsupportedExtension = errors.New("unsupported file extension")
-	ErrEmptySetForCover     = errors.New("no media files available for cover")
-	ErrAlreadyBootstrapped  = errors.New("already bootstrapped")
-	ErrInvalidCredentials   = errors.New("invalid credentials")
-	ErrInvalidFeed          = errors.New("invalid feed")
-	ErrCannotDeleteSelf     = errors.New("cannot delete self")
+	ErrNotFound             = &apiError{msg: "not found", status: http.StatusNotFound}
+	ErrForbidden            = &apiError{msg: "forbidden", status: http.StatusForbidden}
+	ErrShareNotFound        = &apiError{msg: "share not found", status: http.StatusNotFound}
+	ErrMediaNotFound        = &apiError{msg: "media not found", status: http.StatusNotFound}
+	ErrUnsupportedExtension = &apiError{msg: "unsupported file extension", status: http.StatusBadRequest}
+	ErrEmptySetForCover     = &apiError{msg: "no media files available for cover", status: http.StatusBadRequest}
+	ErrAlreadyBootstrapped  = &apiError{msg: "bootstrap already complete", status: http.StatusForbidden}
+	ErrInvalidCredentials   = &apiError{msg: "invalid credentials", status: http.StatusUnauthorized}
+	ErrInvalidFeed          = &apiError{msg: "invalid feed", status: http.StatusBadRequest}
+	ErrCannotDeleteSelf     = &apiError{msg: "cannot delete self", status: http.StatusBadRequest}
+
+	// ErrShareExpired is handled directly by share handlers (not via
+	// handleError); it stays a plain sentinel because no dispatch metadata
+	// is needed.
+	ErrShareExpired = errors.New("share expired")
 )
 
 // MediaQueryFilter defines query parameters for listing media from the API layer.