Add refresh token authentication system

Implement complete refresh token flow for improved security:
- Short-lived access tokens (configurable, default 1h)
- Long-lived refresh tokens (configurable, default 7d)
- Automatic token rotation on refresh
- Token revocation for proper logout

Domain layer:
- Add RefreshToken entity with validation and revocation
- Add RefreshTokenRepository interface

Application layer:
- Add RefreshTokenHandler for token refresh operations
- Add RevokeTokenHandler for single token revocation
- Add RevokeAllTokensHandler for user-wide revocation

Infrastructure layer:
- Implement SQLite RefreshTokenRepository
- Add refresh_tokens table migration with indexes
- Add parseDuration helper for flexible time configuration

HTTP layer:
- Add POST /api/v1/auth/refresh endpoint
- Add POST /api/v1/auth/logout endpoint
- Update login/register to return refresh tokens
- Improve Swagger documentation with clear descriptions

Configuration:
- Update .env.example with secure token expiry defaults
- Add support for minute/hour/day duration formats

Tests:
- Fix test suite to work with new signatures
- All existing tests passing

internal/domain/entities/refresh_token.go

@@ -0,0 +1,34 @@
+package entities
+
+import "time"
+
+type RefreshToken struct {
+	ID        string
+	UserID    string
+	Token     string
+	ExpiresAt time.Time
+	CreatedAt time.Time
+	RevokedAt *time.Time
+}
+
+func NewRefreshToken(userID, token string, expiresAt time.Time) *RefreshToken {
+	return &RefreshToken{
+		UserID:    userID,
+		Token:     token,
+		ExpiresAt: expiresAt,
+		CreatedAt: time.Now(),
+		RevokedAt: nil,
+	}
+}
+
+func (rt *RefreshToken) IsValid() bool {
+	if rt.RevokedAt != nil {
+		return false
+	}
+	return time.Now().Before(rt.ExpiresAt)
+}
+
+func (rt *RefreshToken) Revoke() {
+	now := time.Now()
+	rt.RevokedAt = &now
+}