Signup already captures a click ID per ad network -- gclid, fbclid,
li_fat_id, ttclid, rdt_cid, epik -- but only the UTM parameters reached
the subscription. UTMs say which campaign a customer came from; the click
ID identifies the individual click, which is what Google and Meta need to
match a subscription back to the ad that produced it.
Every key is spelled out at the call site, so what reaches Stripe is
readable in one place without following a constant.
Click IDs are text columns on purpose, since truncating them at 255 would
destroy the value. Stripe caps a metadata value at 500 characters and
rejects the request rather than truncating, so an unbounded column
reaching it would fail the whole checkout and the customer could not
subscribe at all. Values are cut to 500 before they are sent. The UTM
columns are varchar(255) and cannot reach it; this exists for the text
ones.
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