fix(partitioning): render chunk names from the UTC instant

[taran-dbx]

Problem

The partition-manager job failed:

duplicate key value violates unique constraint "idx_chunk_metadata_chunk_name"
DETAIL: Key (chunk_name)=(public.stock_ticks_20260609_040000) already exists.
... ON CONFLICT (chronotable_id, range_start) DO NOTHING

_ensure_partitions builds chunk_name with to_char(range_start, 'YYYYMMDD_HH24MISS') in the session timezone, but range_start is an absolute timestamptz. When the function runs under different session timezones — the scheduled job in UTC vs a client in UTC+2 — the same chunk gets named differently, so chunk_name stops being 1:1 with range_start. A later run then computes a name that collides with a stale row whose range_start differs, and ON CONFLICT (chronotable_id, range_start) can't absorb a chunk_name collision → the insert throws.

Observed on lakets-demo: a row created under UTC+2 stored …_040000 for range_start = 02:00 UTC; the UTC job computing …_040000 for the real 04:00 UTC chunk collided.

Fix

Render the name from the instant in UTC: to_char(v_start AT TIME ZONE 'UTC', 'YYYYMMDD_HH24MISS') (both the CREATE TABLE partition name and the _chunk_metadata.chunk_name). chunk_name is now 1:1 with range_start regardless of session timezone.

Test plan

• make build clean (14 modules, version 0.1.3)
• Reinstall, run partition manager under UTC and a non-UTC session → no chunk_name collision; names match range_start in UTC
• Note: rows created before this fix under a non-UTC session keep their old names; drop stale rows for already-dropped chunks and let the manager recreate them.

Ships as v0.1.3. Reinstall dist/lakets.sql.