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.
boos
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