Here is what "done and correct" means — grade yourself against it. Each line is something the track already taught; the capstone is assembling them without a guide.

Correctness — the feature works:

• A pending signer can decline with a reason; the document becomes declined; the signer's declined state is shown on return.
• Declining is single-use: a second decline (or a sign after a decline) changes zero rows and shows the terminal state, not an error.
• The document status and the audit event are written in one transaction — imagine killing the process mid-handler and confirm neither can land without the other.

Security & integrity — the constraints hold:

• The decline request carries no document id — the token resolves to exactly one signer row (verify a mangled token 404s).
• The document.declined audit event is present, actorSigner, with a frozen message including the reason; it survives even if the document is later deleted (FK-free).
• You did not weaken any existing guard — signing still works, the integrity banner still recomputes, owner routes are still owner-scoped.

Honesty — the framing is intact:

• The UI never implies a decline is more than it is. A decline is evidence that this signer refused these exact bytes at this time — not a legal instrument.

Stretch goals (each re-uses a different lesson, none required):

• Withdraw an invitation (owner-side): let the owner cancel a pending signer before they act — another status='pending'-guarded update, another audit event.
• Decline notifies the owner: reuse the email.Sender interface to email the owner when a signer declines (it prints to your console/logs).
• A terminal-outcome banner on the document detail page summarising completed-vs-declined.
• A GET /documents/{id}/audit.json owner-scoped endpoint returning the trail as JSON — the same authorization rule, a different representation.

Do the core task well before reaching for these. Three solid, correct pieces beat a sprawl of half-wired features — the same standard every lesson held.