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/simulate.py

@@ -26,6 +26,7 @@ from fire_planner.api.schemas import (
     SimulateResult,
 )
 from fire_planner.glide_path import get as get_glide
+from fire_planner.life_events import EventInput, events_to_cashflow_array
 from fire_planner.returns.bootstrap import block_bootstrap
 from fire_planner.returns.shiller import load_from_csv, synthetic_returns
 from fire_planner.scenarios import build_regime_schedule, build_strategy
@@ -47,6 +48,21 @@ def _project(req: SimulateRequest) -> tuple[SimulationResult, float]:
     annual_savings = (np.full(req.horizon_years, float(req.savings_per_year_gbp), dtype=np.float64)
                       if req.savings_per_year_gbp > 0 else None)
     floor = float(req.floor_gbp) if req.floor_gbp is not None else None
+
+    cashflow_adjustments = None
+    if req.life_events:
+        engine_events = [
+            EventInput(
+                year_start=ev.year_start,
+                year_end=ev.year_end,
+                delta_gbp_per_year=float(ev.delta_gbp_per_year),
+                one_time_amount_gbp=(float(ev.one_time_amount_gbp)
+                                     if ev.one_time_amount_gbp is not None else None),
+                enabled=ev.enabled,
+            ) for ev in req.life_events
+        ]
+        cashflow_adjustments = events_to_cashflow_array(engine_events, req.horizon_years)
+
     started = time.perf_counter()
     result = simulate(
         paths=paths,
@@ -57,6 +73,7 @@ def _project(req: SimulateRequest) -> tuple[SimulationResult, float]:
         regime=build_regime_schedule(req.jurisdiction, req.leave_uk_year),
         horizon_years=req.horizon_years,
         annual_savings=annual_savings,
+        cashflow_adjustments=cashflow_adjustments,
     )
     elapsed = time.perf_counter() - started
     return result, elapsed