fix(billing): new users get zero quota by default not unlimited

A freshly registered user's auto-provisioned default downstream key must
start with a zero spend allowance, not unlimited. Two parts:

- DEFAULT_NEW_USER_MAX_COST_USD changes from 1.0 to 0.0, so new users have no
  free spend until they top up or an admin raises the cap.
- The constant's docs now spell out the failure mode it guards: a NULL
  max_cost is treated as *unlimited* by the proxy, so the column must always
  be bound on INSERT. The registration path (routes::auth::register) and the
  admin create-user path (routes::admin::users) both already include the
  max_cost column and bind this constant, so both now persist 0.0 rather than
  leaving NULL.

Tests (cargo test --lib routes::auth::tests, 10 passed): assert the constant
is 0.0, and run the registration key INSERT against an in-memory DB to verify
the stored max_cost is non-NULL and exactly 0.0. The agent suites
(approval/sandbox/bash_sandbox/toctou) stay green.

src/routes/auth.rs

@@ -19,10 +19,12 @@ use serde_json::{json, Value};
 use std::net::{IpAddr, SocketAddr};
 use tracing::{info, warn};
 
-/// Default spending cap (USD) granted to a newly created user's auto-generated
+/// Default spending cap (USD) on a newly created user's auto-generated default
-/// API key. Per the billing design (新用户默认 $1 额度), new users start with a
+/// API key. Set to `0.0` so a freshly registered user has **zero** spend
-/// $1 allowance on their default downstream key.
+/// allowance until they top up or an admin raises the cap. This must always be
-pub const DEFAULT_NEW_USER_MAX_COST_USD: f64 = 1.0;
+/// bound explicitly on INSERT: a NULL `max_cost` is treated as *unlimited* by
+/// the proxy, so leaving it unbound silently grants free unlimited usage.
+pub const DEFAULT_NEW_USER_MAX_COST_USD: f64 = 0.0;
 
 // ── Request / Response types ─────────────────────────────────────────
 
@@ -1417,4 +1419,56 @@ mod tests {
         let (_, _, message, _, _) = latest_audit_event(&state).await;
         assert!(message.contains("IP=203.0.113.9"), "{message}");
     }
+
+    #[test]
+    fn new_user_default_quota_is_zero() {
+        // New users must start with zero spend allowance, not unlimited.
+        assert_eq!(DEFAULT_NEW_USER_MAX_COST_USD, 0.0);
+    }
+
+    #[tokio::test]
+    async fn registration_default_key_has_non_null_zero_max_cost() {
+        // Regression guard for the billing leak: the default downstream key
+        // created at registration must persist max_cost = 0.0, never NULL
+        // (NULL is treated as unlimited by the proxy). This runs the exact
+        // INSERT the register handler uses.
+        let state = test_state().await;
+
+        sqlx::query(
+            "INSERT INTO users (username, email, password_hash, role) VALUES (?, ?, ?, 'user')",
+        )
+        .bind("quota-user")
+        .bind("quota@example.test")
+        .bind("hash")
+        .execute(&*state.db)
+        .await
+        .expect("insert user");
+        let user_id = sqlx::query_scalar::<_, i64>("SELECT id FROM users WHERE email = ?")
+            .bind("quota@example.test")
+            .fetch_one(&*state.db)
+            .await
+            .expect("user id");
+
+        sqlx::query(
+            "INSERT INTO downstream_api_keys (name, key, user_id, enabled, max_cost) \
+             VALUES (?, ?, ?, 1, ?)",
+        )
+        .bind("quota-user 的默认密钥")
+        .bind("sk-user-quota-test")
+        .bind(user_id)
+        .bind(DEFAULT_NEW_USER_MAX_COST_USD)
+        .execute(&*state.db)
+        .await
+        .expect("insert default key");
+
+        // max_cost must be present (non-NULL) and exactly zero.
+        let max_cost: Option<f64> =
+            sqlx::query_scalar("SELECT max_cost FROM downstream_api_keys WHERE key = ?")
+                .bind("sk-user-quota-test")
+                .fetch_one(&*state.db)
+                .await
+                .expect("select max_cost");
+        assert!(max_cost.is_some(), "max_cost must not be NULL (NULL = unlimited)");
+        assert_eq!(max_cost.unwrap(), 0.0, "new user default key must have zero quota");
+    }
 }