Replaced the previous signature persistence mechanism with
`IMemoryCache` for temporary storage of captured signatures
using a unique `Guid` key and a 1-minute TTL. Added logging
to track cached signatures and their associated envelope keys.
Removed the logic for rebuilding and displaying reports with
overlaid signatures. Instead, implemented navigation to a
new signed page (`/envelope/{EnvelopeKey}/signed`) with the
signature ID passed as a query parameter.
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