اضافه شدن قابلیت share و نگهداری بلندمدت

backend/app/expiry.py

@@ -0,0 +1,15 @@
+"""Link expiry policy. -1 is a persistent no-expiry sentinel, not a timestamp."""
+from fastapi import HTTPException
+from .config import settings
+
+NEVER_EXPIRES = -1
+
+
+def expiration(expires_in, now):
+    if expires_in == NEVER_EXPIRES:
+        return NEVER_EXPIRES
+    ttl = settings().link_ttl if expires_in is None else expires_in
+    if ttl <= 0 or ttl > settings().link_ttl:
+        raise HTTPException(422, {'code': 'invalid_expiry',
+                                 'message': 'expires_in must be -1 (no expiry) or positive seconds within the configured lifetime'})
+    return now + ttl