fix(auth): re-add session table model mapping

Better-Auth defaults to singular "session" table with camelCase
columns, but our DB uses plural "sessions" with snake_case columns
(migration 002). Add modelName and field mappings to match the
existing pattern for user, account, and verification models.

Fixes: 401 on login and 422 on registration.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

auth/src/auth.ts

@@ -36,6 +36,15 @@ export const auth = betterAuth({
   },
 
   session: {
+    modelName: "sessions",
+    fields: {
+      userId: "user_id",
+      expiresAt: "expires_at",
+      ipAddress: "ip_address",
+      userAgent: "user_agent",
+      createdAt: "created_at",
+      updatedAt: "updated_at",
+    },
     expiresIn: 60 * 60 * 24 * 7, // 7 days
     updateAge: 60 * 60 * 24, // refresh after 1 day
     cookieCache: {