feat: add ai_user_daily_spend table and queries (#26562)

## Description

Adds the spend tracking table and queries needed by [AIGOV-427](https://linear.app/codercom/issue/AIGOV-427/add-post-response-spend-accumulation) (post-response accumulation) and [AIGOV-428](https://linear.app/codercom/issue/AIGOV-428/add-pre-request-budget-enforcement) (pre-request enforcement).

## Changes

- Add `ai_user_daily_spend` table to aggregate per-user, per-effective-group AI spend by UTC day.
- Add `UpsertUserAIDailySpend` and `GetUserAISpendSince` queries.

Closes https://linear.app/codercom/issue/AIGOV-426/add-daily-spend-table-and-queries

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

coderd/database/migrations/testdata/fixtures/000536_ai_user_daily_spend.up.sql

@@ -0,0 +1,7 @@
+INSERT INTO ai_user_daily_spend (
+    user_id,
+    effective_group_id,
+    day,
+    spend_micros
+) VALUES
+    ('30095c71-380b-457a-8995-97b8ee6e5307', 'bb640d07-ca8a-4869-b6bc-ae61ebb2fda1', '2024-06-15', 100000);