fix(ca): translate sql.ErrNoRows to nil,nil for optional lookups

Repository methods that perform single-row queries now return (nil,nil)
instead of leaking sql.ErrNoRows when a row is missing. This aligns with
service-layer expectations (e.g. GetMediaDetail, ValidateSession,
UpdateProgress, AssignTag, access checks) so normal missing data does
not break app flows.

Files changed:
- internal/repository/media.go, user.go, set.go, set_permission.go,
  tag.go, note.go, playback_progress.go, playback_accumulator.go,
  session.go, share.go
- internal/repository/sqlite_test.go (updated assertions)
- internal/repository/sqlite_no_rows_test.go (new focused repository tests)
- internal/service/no_rows_test.go (new focused service tests)

1 files changed, 4 insertions, 0 deletions

diff --git a/internal/repository/user.go b/internal/repository/user.go
index 7f366c2..073f61d 100644
--- a/internal/repository/user.go
+++ b/internal/repository/user.go
@@ -8,6 +8,7 @@ import (
 	"github.com/paul/kiss-media-player/internal/model"
 )
 
+
 // CreateUser inserts a new user and returns the generated ID.
 func (s *SQLite) CreateUser(ctx context.Context, user *model.User) (int64, error) {
 	res, err := s.db.ExecContext(ctx,
@@ -24,6 +25,9 @@ func scanUser(row sqlScanner) (*model.User, error) {
 	var u model.User
 	var admin int
 	err := row.Scan(&u.ID, &u.Username, &u.PasswordHash, &admin, &u.CreatedAt)
+	if err == sql.ErrNoRows {
+		return nil, nil
+	}
 	if err != nil {
 		return nil, err
 	}