Every channel-bearing INSERT INTO events now checks created_at against
database clock_timestamp() minus CREATED_AT_FLOOR_SECS on the admitted
transaction before inserting, and returns the typed, non-retryable
DbError::CreatedAtFloorRejected. The check samples the clock after BEGIN,
so the replica-fence wall min(oldest_xact_start, S) - floor - margin
still covers it. Migration 0021's deferred trigger and the writer-pool
GUC stay in place as the mixed-version backstop until phase 2.
A source policy requires the check on every channel-bearing events
INSERT and forbids production UPDATEs of events.created_at/channel_id.
The admin seed script now refuses non-local PGHOST.
Refs BUZZ-316.
Signed-off-by: Elrond <28d6302a099e5225b02c4155ac4236e4912603df2ab08dbfc2f4fef08ce598c8@buzz.block.builderlab.xyz>
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