feat: schedule preview and off-cycle payouts

Two things an operator needs that the scheduler alone does not give them.

Preview draws a contract's calendar from loose terms — start date,
frequency, period count — with no employee or wallet required, because the
schedule is what an operator wants to sanity-check first and a mistyped
start date is cheapest to fix before anything is saved. The same shape is
available for a live contract, from its current position. Both return
`ends_on`: "12 monthly payments from 15 Jan" is far easier to verify
against "ends 15 Dec" than against a list of twelve dates.

pay-now settles the next period immediately and consumes it. Deliberately
one endpoint rather than two: "run it now, don't wait for the tick" and
"pay it early" are the same operation and differ only in whether today
happens to be the payday. It bypasses the back-dated skip — that guard
exists to stop a new contract firing surprise back-pay, and an operator
explicitly asking to pay is not a surprise — and it takes the same
per-contract lock as the scheduler, which is what that lock was added for.
The ledger row is returned even on failure, since a manual payout that did
not land is exactly when you want the reason.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018jy52j9GRZ6XKa1Zt21LLj

models.py

@@ -244,3 +244,41 @@ class Payout(BaseModel):
     @property
     def amount_sat(self) -> int | None:
         return None if self.amount_msat is None else self.amount_msat // 1000
+
+
+# ---------------------------------------------------------------------------
+# Schedule preview
+# ---------------------------------------------------------------------------
+
+
+class SchedulePreviewRequest(BaseModel):
+    """Enough of a contract to draw its calendar, with none of its identity.
+
+    Deliberately not a `CreateContract`: previewing must work before an
+    employee or a wallet has been chosen, since the schedule is usually the
+    thing the operator wants to sanity-check first.
+    """
+
+    start_date: str  # YYYY-MM-DD
+    frequency: Frequency = Frequency.monthly
+    total_periods: int | None = None
+    count: int = 12  # how many paydays to draw
+
+    @validator("start_date")
+    def _valid_date(cls, v: str) -> str:
+        datetime.strptime(v, "%Y-%m-%d")
+        return v
+
+    @validator("count")
+    def _sane_count(cls, v: int) -> int:
+        return max(1, min(v, 120))
+
+
+class SchedulePreview(BaseModel):
+    paydays: list[str]  # YYYY-MM-DD, in order
+    total_periods: int | None = None
+    # The contract's last payday, or None for an open-ended one. Worth
+    # surfacing on its own: "12 monthly payments from 15 Jan" is much easier
+    # to check against "ends 15 Dec" than against a list.
+    ends_on: str | None = None
+    truncated: bool = False  # more paydays exist than were drawn