Fix SQLite timestamp millisecond formatting to always include 3 digits

[brandur]

Use a .000 Go time layout so SQLite timestamps always contain exactly
three fractional digits. This matches datetime('now', 'subsec') and
prevents equal instants from being represented differently as .1 and
.100.

In Go, time layouts work like this:

• .999 omits trailing zeros, producing values like .1
• .000 always emits three digits, producing .100

This was always a preexisting problem, but Codex happened to catch it
while implementing SQLite support in River Pro, and we may as well fix
it here too.