docs: note the scheduled_at future/30-day window

README.md

@@ -126,7 +126,7 @@ sk.send(
     cc=["ops@example.com"],                              # email only
     from_="hello@acme.com",                              # email only — From address override (bare address)
     from_name="Acme Support",                            # email only — From display name → "Acme Support <hello@acme.com>"
-    scheduled_at=datetime(2026, 1, 1, 9, 0, tzinfo=timezone.utc),  # datetime or ISO-8601 string
+    scheduled_at=datetime(2026, 1, 1, 9, 0, tzinfo=timezone.utc),  # datetime or ISO-8601 string; future & ≤ 30 days ahead
     idempotency_key="order-1234-shipped",                # optional; see Idempotency below
 )
 ```

src/senderkit/models.py

@@ -107,6 +107,8 @@ class TemplateSend:
     version: Optional[int] = None
     channel: Optional[ChannelLike] = None
     metadata: Optional[Metadata] = None
+    #: Defer delivery. Must be in the future and at most 30 days ahead; the API
+    #: rejects a past or too-distant timestamp with ``400``. Omit to send now.
     scheduled_at: Optional[ScheduledAt] = None
     cc: Optional[List[str]] = None
     bcc: Optional[List[str]] = None
@@ -134,6 +136,8 @@ class RawSend:
     interpolate: Optional[bool] = None
     vars: Optional[Vars] = None
     metadata: Optional[Metadata] = None
+    #: Defer delivery. Must be in the future and at most 30 days ahead; the API
+    #: rejects a past or too-distant timestamp with ``400``. Omit to send now.
     scheduled_at: Optional[ScheduledAt] = None
     idempotency_key: Optional[str] = None