refactor: make user AI budget and spend endpoints consistent (#27611)

## Description

Makes the user AI cost control endpoints consistent.

## Changes

- Replaces the flat `spend_limit_micros` and `limit_source` fields on
`GET /users/{user}/ai/spend` with a nested `effective_budget`, reusing
the type behind `group_budget`. The flat pair made it possible to encode
a limit without a source.
- Renames `AIGroupBudget` to `AIBudgetLimit`, since it also carries
`user_override` limits and is no longer group-specific. The type name is
not part of the wire format.
- Moves `/users/{user}/ai/budget` to `/users/{user}/ai/budget/override`.
The endpoint only ever managed the per-user override, which the type,
the handlers, and the operation IDs all already said; the path was the
only place that didn't.

> [!NOTE]
> Initially generated by Claude Opus 4.7, modified and reviewed by
@ssncferreira

coderd/database/db2sdk/db2sdk.go

@@ -1490,7 +1490,7 @@ func GroupMemberAISpend(row database.GetGroupMembersAISpendRow) codersdk.GroupMe
 		member.EffectiveGroupID = &row.EffectiveGroupID.UUID
 	}
 	if row.SpendLimitMicros.Valid {
-		member.GroupBudget = &codersdk.AIGroupBudget{
+		member.GroupBudget = &codersdk.AIBudgetLimit{
 			SpendLimitMicros: row.SpendLimitMicros.Int64,
 			LimitSource:      codersdk.AIBudgetLimitSource(row.LimitSource.String),
 		}