fix(subsonic): getUser answers an admin's lookup with the requested user (#6292)

An admin asking for another username got their own name and roles back.
The handler now looks the user up by exact username and answers with
that user's payload; an unknown username is ErrDataNotFound. A
non-admin asking about someone else is still ErrNotAuthorized.

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

internal/subsonic/user.go

@@ -1,9 +1,13 @@
 package subsonic
 
 import (
+	"context"
 	"encoding/xml"
+	"errors"
 	"net/http"
 
+	"github.com/jackc/pgx/v5"
+
 	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
 )
 
@@ -38,26 +42,41 @@ type User struct {
 	VideoConversionRole bool     `json:"videoConversionRole" xml:"videoConversionRole,attr"`
 }
 
-// handleGetUser returns the authenticated user when no username is provided
-// or when the requested username matches; admins may look up any user. Per
-// spec a non-admin asking about another user gets ErrNotAuthorized.
-func handleGetUser(w http.ResponseWriter, r *http.Request) {
+// userHandlers serves getUser. lookup finds a user by exact username; it is
+// a field so tests can answer without a database.
+type userHandlers struct {
+	lookup func(ctx context.Context, username string) (dbq.User, error)
+}
+
+// getUser returns the authenticated user when no username is provided or
+// when the requested username matches; admins may look up any user, and an
+// unknown username is ErrDataNotFound. Per spec a non-admin asking about
+// another user gets ErrNotAuthorized.
+func (h *userHandlers) getUser(w http.ResponseWriter, r *http.Request) {
 	caller, ok := UserFromContext(r.Context())
 	if !ok {
 		WriteFail(w, r, ErrNotAuthorized, "Not authenticated")
 		return
 	}
 	requested := r.URL.Query().Get("username")
-	if requested != "" && requested != caller.Username && !caller.IsAdmin {
+	if requested == "" || requested == caller.Username {
+		Write(w, r, UserResponse{Envelope: NewEnvelope("ok"), User: userPayload(caller)})
+		return
+	}
+	if !caller.IsAdmin {
 		WriteFail(w, r, ErrNotAuthorized, "Not authorized to view other users")
 		return
 	}
-	// No per-user lookup: an admin asking for another username is answered
-	// with the caller's own roles.
-	Write(w, r, UserResponse{
-		Envelope: NewEnvelope("ok"),
-		User:     userPayload(caller),
-	})
+	target, err := h.lookup(r.Context(), requested)
+	if errors.Is(err, pgx.ErrNoRows) {
+		WriteFail(w, r, ErrDataNotFound, "User not found")
+		return
+	}
+	if err != nil {
+		WriteFail(w, r, ErrGeneric, "Failed to look up user")
+		return
+	}
+	Write(w, r, UserResponse{Envelope: NewEnvelope("ok"), User: userPayload(target)})
 }
 
 func userPayload(u dbq.User) User {