Feature: Account-level weekly cost limit (weekly_cost_limit) for Anthropic accounts

[xsha511]

Background

Claude Max subscriptions have two rate-limit windows:

• 5-hour window — short-term usage cap
• 7-day window — weekly usage cap

Sub2API currently supports proactive cost control only for the 5h window via window_cost_limit in the account's extra field. For the 7d window, Sub2API can only reactively handle 429 responses by parsing anthropic-ratelimit-unified-7d-utilization and anthropic-ratelimit-unified-7d-reset headers.

Problem

There is no way to proactively set a weekly cost limit per account. Administrators cannot configure "stop scheduling this account after it spends $X this week." The only option is to wait until Anthropic returns a 429 with 7d window exhaustion, which means:

1. The account has already hit its hard limit — no budget headroom control
2. No ability to set a conservative weekly budget below Anthropic's hard cap
3. Cannot distribute weekly quota evenly across accounts (some accounts may burn through their weekly allowance early in the week)

Adjusting window_cost_limit to indirectly control weekly spend is imprecise (168h / 5h = ~33 windows, hard to predict actual usage distribution).

Proposed Solution

Add a weekly_cost_limit field to the account extra configuration, similar to how window_cost_limit works:

{
  "window_cost_limit": 50,
  "window_cost_sticky_reserve": 10,
  "weekly_cost_limit": 200,
  "weekly_cost_sticky_reserve": 20
}

Scheduling behavior (same three-state pattern as window cost):

Current weekly cost	State	Behavior
< weekly_cost_limit	Schedulable	Accept new requests normally
weekly_cost_limit ~ weekly_cost_limit + sticky_reserve	StickyOnly	Only allow existing sticky sessions
>= weekly_cost_limit + sticky_reserve	NotSchedulable	Fully stop scheduling

Implementation considerations:

• Weekly window start could align with Anthropic's 7d window (rolling 7 days) or use a fixed weekly reset (e.g., Monday 00:00 UTC)
• Cost calculation should reuse the existing usageLogRepo.GetAccountWindowStats() pattern with a 7-day lookback
• Can be integrated into the existing isAccountSchedulableForWindowCost() filter chain in GatewayService

Use Case

Operators managing multiple Claude Max subscription accounts who want to:

• Set conservative weekly budgets below Anthropic's hard limit
• Evenly distribute usage across accounts throughout the week
• Prevent any single account from consuming its entire weekly quota early