engine+api: plumb life events into the simulator

Until now life events were stored but ignored by the engine — pure
metadata. Now they actually move portfolios.

Engine:
- simulator.simulate() takes optional cashflow_adjustments: a (n_years,)
  real-GBP array applied each year *after* savings + return but
  *before* withdrawal. Positive = inflow, negative = outflow.
- New fire_planner/life_events.py with EventInput dataclass +
  events_to_cashflow_array(events, horizon). Handles ranged deltas,
  one-time amounts, disabled events, year clipping past horizon,
  negative year_start (clipped to 0), and summing multiple events.

API:
- /simulate accepts optional life_events list. Server converts each
  to EventInput, builds cashflow_adjustments, passes to simulate().
- Frontend Run-now on scenario detail now fetches the scenario's
  life events and includes them in the request — projections finally
  reflect "retire at 50, kid born at y3, inheritance at y22".

Tests: 11 events helper + 4 end-to-end engine + 1 API integration =
16 new tests. 188 total (was 172). mypy strict + ruff clean.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

fire_planner/api/schemas.py

@@ -203,6 +203,15 @@ class GoalCreate(BaseModel):
 # ── simulate / compare ───────────────────────────────────────────────
 
 
+class LifeEventInput(BaseModel):
+    """Engine-level event shape — same as the DB row's relevant fields."""
+    year_start: int = Field(ge=0, le=100)
+    year_end: int | None = Field(default=None, ge=0, le=100)
+    delta_gbp_per_year: Decimal = Decimal("0")
+    one_time_amount_gbp: Decimal | None = None
+    enabled: bool = True
+
+
 class SimulateRequest(BaseModel):
     """Sync, non-persisted simulate. Used by the React UI for what-if."""
     jurisdiction: str
@@ -216,6 +225,7 @@ class SimulateRequest(BaseModel):
     floor_gbp: Decimal | None = None
     n_paths: int = Field(ge=100, le=50_000, default=5_000)
     seed: int = 42
+    life_events: list[LifeEventInput] = Field(default_factory=list)
 
 
 class SimulateResult(BaseModel):