fix: use sqlx chrono decode for thread timestamps instead of manual parsing

[matt2e]

Summary

Use sqlx's native DateTime<Utc> decoding for thread timestamp columns (created_at, updated_at, archived_at) in ThreadRow instead of fetching as String and manually parsing with .parse().

Problem

The current code fetches timestamps as raw strings and parses them manually:

created_at: created_at.parse().unwrap_or_else(|_| Utc::now()),
updated_at: updated_at.parse().unwrap_or_else(|_| Utc::now()),

This has two issues:

1. On parse failure, it silently falls back to Utc::now(), producing incorrect timestamps
2. It duplicates work that sqlx already handles natively via the chrono feature

Fix

Change ThreadRow to decode timestamps as DateTime<Utc> directly, and simplify thread_from_row to pass them through without parsing.

Context

This fix was originally merged into the baxen/goose2 branch (PR #8502) but was missed when goose2 was moved into main via PR #8516 — the move PR was prepared from a snapshot that predated #8502.