CRITICAL: Add replay protection for authorization codes

Authorization codes were implemented as stateless JWTs with no tracking,
allowing the same code to be exchanged for tokens multiple times. This
violates OAuth 2.0 RFC 6749 Section 4.1.2 which mandates that authorization
codes MUST be single-use.

This change:
- Adds oidc_authorization_codes table to track code usage
- Stores authorization codes in database when generated
- Validates code exists and hasn't been used before exchange
- Marks code as used immediately after validation
- Prevents replay attacks where intercepted codes could be reused

Security impact:
- Prevents attackers from reusing intercepted authorization codes
- Ensures compliance with OAuth 2.0 security requirements
- Adds database-backed single-use enforcement

internal/model/oidc_authorization_code_model.go

@@ -0,0 +1,15 @@
+package model
+
+type OIDCAuthorizationCode struct {
+	Code        string `gorm:"column:code;primaryKey"`
+	ClientID    string `gorm:"column:client_id;not null"`
+	RedirectURI string `gorm:"column:redirect_uri;not null"`
+	Used        bool   `gorm:"column:used;default:false"`
+	ExpiresAt   int64  `gorm:"column:expires_at;not null"`
+	CreatedAt   int64  `gorm:"column:created_at;not null"`
+}
+
+func (OIDCAuthorizationCode) TableName() string {
+	return "oidc_authorization_codes"
+}
+