Improve OAuth audit log messages with provider name and user identity

Use config.oauth.providerName (e.g. "Keycloak", "Google") instead of
the raw provider ID "oauth2" in audit summaries. Include user name or
email in sign-in and sign-up messages for easier log reading.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/lib/auth.ts

@@ -231,7 +231,7 @@ export const { handlers, signIn, signOut, auth } = NextAuth({
                     action: "account_linked",
                     entityType: "user",
                     entityId: updatedUser.id,
-                    summary: `OAuth account linked while authenticated: ${account.provider}`,
+                    summary: `OAuth account linked via ${config.oauth.providerName}`,
                     data: JSON.stringify({ provider: account.provider, email: user.email })
                   });
 
@@ -262,7 +262,7 @@ export const { handlers, signIn, signOut, auth } = NextAuth({
             action: "oauth_signin",
             entityType: "user",
             entityId: existingOAuthUser.id,
-            summary: `User signed in via ${account.provider}`,
+            summary: `${existingOAuthUser.name || existingOAuthUser.email || "User"} signed in via ${config.oauth.providerName}`,
             data: JSON.stringify({ provider: account.provider })
           });
 
@@ -298,7 +298,7 @@ export const { handlers, signIn, signOut, auth } = NextAuth({
                 action: "account_linked",
                 entityType: "user",
                 entityId: linkedUser.id,
-                summary: `OAuth account auto-linked: ${account.provider}`,
+                summary: `OAuth account auto-linked via ${config.oauth.providerName}`,
                 data: JSON.stringify({ provider: account.provider, email: user.email })
               });
 
@@ -341,7 +341,7 @@ export const { handlers, signIn, signOut, auth } = NextAuth({
           action: "oauth_signup",
           entityType: "user",
           entityId: newUser.id,
-          summary: `New user created via ${account.provider} OAuth`,
+          summary: `New user ${user.name || user.email || ""} created via ${config.oauth.providerName}`,
           data: JSON.stringify({ provider: account.provider, email: user.email })
         });