### Check for existing issues

- [x] I have searched the existing issues and checked that my issue is not a duplicate.

### The Feature

## Proposal

Make partition management timezone-safe. Several changes reinforce each other; any one of them would eliminate our symptom, but the combination is what makes the design correct.

1. **Change the column type** — annotate `LiteLLM_SpendLogs.startTime` (and `endTime`, `completionStartTime`) with `@db.Timestamptz` in `schema.prisma`. Absolute-time storage removes the whole class of session-TZ bugs. 2. **Pin the cleanup session to UTC** — issue `SET LOCAL timezone = 'UTC'` at the top of the transaction that runs `ensure_partitions` / `drop_partitions_older_than` / the retention DELETE. Cheap, backward-compatible, and defends against operators who keep the DB in a non-UTC zone. 3. **Emit tz-explicit partition boundary strings** — `FOR VALUES FROM ('2026-07-17 00:00:00+00') TO ('2026-07-18 00:00:00+00')` instead of bare dates, so the bounds are unambiguous regardless of session TZ.

### Motivation, pitch

## What we observed in production

Our PostgreSQL cluster runs with `timezone = 'Asia/Seoul'` (KST). LiteLLM cleanup on 2026-07-23 00:00 K…