Go does not URL-decode Cookie.Value automatically, so the frontend's
JSON-then-encodeURIComponent payload was landing in PostHog as
percent-encoded garbage (%7B%22utm_source...). Unescape on read so the
backend receives the original JSON string the frontend intended, and
drop values that fail to decode or exceed the server-side cap — sending
truncated garbage is worse than sending nothing. Oversized-cookie guard
matches the frontend's SIGNUP_SOURCE_MAX_LEN.