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>

tests/test_api_simulate.py

@@ -113,6 +113,42 @@ async def test_compare_runs_two_scenarios(client: AsyncClient) -> None:
     assert all(len(r["yearly"]) == 20 for r in results)
 
 
+async def test_simulate_with_life_events_changes_outcome(client: AsyncClient) -> None:
+    """Same params with vs without a £500k inheritance at year 5 — the
+    inheritance run must end with strictly more median NW."""
+    base_req = {
+        "jurisdiction": "uk",
+        "strategy": "trinity",
+        "leave_uk_year": 0,
+        "glide_path": "static_60_40",
+        "spending_gbp": "60000",
+        "nw_seed_gbp": "1500000",
+        "horizon_years": 30,
+        "n_paths": 200,
+        "seed": 42,
+    }
+    base = await client.post("/simulate", json=base_req)
+    assert base.status_code == 200, base.text
+
+    enhanced = await client.post(
+        "/simulate",
+        json={
+            **base_req,
+            "life_events": [
+                {
+                    "year_start": 5,
+                    "one_time_amount_gbp": "500000",
+                }
+            ],
+        },
+    )
+    assert enhanced.status_code == 200, enhanced.text
+
+    base_p50 = float(base.json()["p50_ending_gbp"])
+    enhanced_p50 = float(enhanced.json()["p50_ending_gbp"])
+    assert enhanced_p50 > base_p50
+
+
 async def test_compare_rejects_single_scenario(client: AsyncClient) -> None:
     resp = await client.post(
         "/compare",