refactor: narrow auth service to smaller units

Easier for testing, and omits need for full dep injection for pusher service, where we passed in nil for deps dangerously.

go/internal/auth/reader.go

@@ -0,0 +1,45 @@
+package auth
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+
+	"github.com/redis/go-redis/v9"
+)
+
+type SessionReader interface {
+	// GetSession returns ErrSessionNotFound if no valid session.
+	GetSession(ctx context.Context, sessionToken string) (*Session, error)
+}
+
+type sessionReaderImpl struct {
+	redisClient *redis.Client
+}
+
+// newSessionReader returns the concrete reader. Used by NewAuthService to
+// embed without going through the SessionReader interface (which would hide
+// redisClient from the rest of authServiceImpl).
+func newSessionReader(redisClient *redis.Client) *sessionReaderImpl {
+	return &sessionReaderImpl{redisClient: redisClient}
+}
+
+func NewSessionReader(redisClient *redis.Client) SessionReader {
+	return newSessionReader(redisClient)
+}
+
+func (r *sessionReaderImpl) GetSession(ctx context.Context, token string) (*Session, error) {
+	sessionInfo, err := r.redisClient.Get(ctx, token).Result()
+	if err != nil {
+		if errors.Is(err, redis.Nil) {
+			return nil, ErrSessionNotFound
+		}
+		return nil, fmt.Errorf("error getting session: %w", err)
+	}
+
+	var session Session
+	err = json.Unmarshal([]byte(sessionInfo), &session)
+
+	return &session, nil
+}