## Description
Adds pre-request AI budget enforcement to `aibridged`. Requests are rejected with HTTP 403 when the user's aggregated spend for the current period has reached their effective limit.
## Changes
- Add `IsBudgetExceeded` RPC to `aibridgedserver`. Resolves the user's effective budget, aggregates spend over the caller-supplied `[period_start, now]` window, and returns whether the limit has been reached along with the effective limit.
- Wire the check into `aibridged`'s HTTP handler. The caller computes the period start (monthly for now) and passes it in the request.
- Reject exceeded requests with HTTP 403 Forbidden and a message directing the user to contact an administrator.
- Add `dbtime.StartOfMonth` alongside `StartOfDay` for period computation.
- Add real-DB tests covering the enforcement path: month-boundary excludes prior-period spend, and a new user override unblocks a previously-exceeded user.
Closes https://linear.app/codercom/issue/AIGOV-428/add-pre-request-budget-enforcement
> [!NOTE]
> Initially generated by Claude Opus 4.7, modified and reviewed by @ssncferreira
Susana Ferreira