feat(09-01): add theme column to user table + Better Auth additionalField

- migrations/084_add_user_theme.sql: ALTER TABLE user ADD COLUMN theme TEXT NOT NULL DEFAULT 'system'
- Defensive backfill UPDATE for in-flight NULL rows on managed Postgres
- COMMENT ON COLUMN documents allowed values (light|dark|system)
- lib/auth.ts: adds theme additionalField with defaultValue 'system' after timezone
- session.user.theme now exposed via Better Auth same as session.user.timezone

lib/auth.ts

@@ -96,6 +96,10 @@ export const auth = betterAuth({
         type: "string",
         defaultValue: process.env.DEFAULT_TIMEZONE || "UTC",
       },
+      theme: {
+        type: "string",
+        defaultValue: "system",
+      },
     },
   },
 });