Fix bugs in chat blast nofis

[dharit-tan]

Description

This fixes 2 bugs in the blast notifs:

• js dates have 3 decimal places less precision than postgres dates, so truncate the postgres dates when comparing them to js values.
• We were getting many duplicate notifs being sent to the same user so add a DISTINCT clause to the sql query. This indicates something might be messed up in the query, but this fixes for now.
• In the case where all recipients of a blast already have a 1-1 chat thread with the sender, we will get no outgoing messages. Previous design considered this to be the "we have reached the end of blasts" case, but in fact we should explicitly check if there are any blasts newer than the current one.

Also added a log to improve observability

How Has This Been Tested?

Tested by making these edits on stage DN 1 and observing logs. Will write a test to follow-up.

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Latest commit: aad0ac3bbf10f62472ff726e52a0920756664eaf

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