## Description
Read the AI budget period from the deployment config on both the RPC server and the `/users/{user}/ai/spend` endpoint, instead of hardcoding `month`. Drops the `period_start` RPC parameter that was incorrectly introduced in #26915: the period should have been derived from the deployment config from the start.
## Changes
- Add `codersdk.NewAIBudgetPeriodFromString`, mirroring `NewAIBudgetPolicyFromString`.
- `aibridgedserver.Server` takes a `quartz.Clock`, reads `BudgetPeriod` from the deployment config at construction, and computes the period window inside `IsBudgetExceeded`.
- Remove `period_start` from `IsBudgetExceededRequest` and stop sending it from the daemon.
- The `userAISpendStatus` endpoint reads the period from `AIBridgeConfig.BudgetPeriod` instead of hardcoding month.
> [!NOTE]
> Initially generated by Claude Opus 4.7, modified and reviewed by @ssncferreira
Susana Ferreira
6580cdcf7f