bmersereau puts a clock on download links in willchen96's Mike

Every signed download link in this fork now carries a built-in expiry - 30 days by default. After that, the link stops working, even if someone still has the URL sitting in a chat thread or email. The window is chosen to be long enough that normal sharing keeps working, but short enough that a compromised signing key doesn't hand an attacker a permanent backdoor to every file ever shared.

The rollout is deliberately gentle. Links minted before this change keep working indefinitely, so nothing breaks the day the code ships - the new safety net only applies going forward. bmersereau also stood up a proper test harness for this corner of the backend, with coverage for tampered links, expired links, and the happy path.