Device clocks on MeshCore nodes are wildly inaccurate (off by hours or
epoch-near values like 4). The channel messages endpoint was using
sender_timestamp as part of the deduplication key, which could cause
messages to fail deduplication or incorrectly collide.
Changed dedupe key from sender:timestamp to sender:hash, which is the
correct unique identifier for a transmission.
Also added TIMESTAMP-AUDIT.md documenting all device timestamp usage.