fire-planner: What-If gains the chart-first scenario editor

The Plan-tab editors (interactive Gantt for life events, flex spending
rules) are now available in What-If too — with local state instead of
DB persistence so users can tweak before committing to a scenario.

Architecture refactor:

- EventGantt is now a controlled component. The `scenarioId` prop +
  internal useMutation/useQueryClient hooks went away; the component
  takes a `persister: { create, patch, delete }` prop and delegates
  every mutation through it. Plan tab wires it to lifeEventsApi +
  cache invalidation; What-If wires it to React local state with
  negative ids for new events.
- FlexRulesEditor is similarly controlled. Takes `rules + onChange`
  instead of a `scenario` object. Plan tab wraps it with the PATCH
  /scenarios/:id mutation; What-If wraps it with setFlexRules.

Backend:

- New stateless POST /scenarios/spending-profile-preview endpoint
  takes base_spending_gbp + horizon + life_events + flex_rules in the
  body and returns the same SpendingProfileResponse shape as the
  read-only /scenarios/{id}/spending-profile endpoint. Used by
  What-If to render the stacked-area chart against unsaved events.
- SpendingProfileResponse.scenario_id loosened to int | None to
  support the preview variant.

Frontend:

- What-If page gains `events` + `flexRules` local state, an
  EventGantt + FlexRulesEditor wired through them, and a Visx
  spending-profile chart fed by /spending-profile-preview.
- Sim auto-refresh: a 600ms debounced effect re-fires /simulate
  whenever the form / events / flex rules change. Manual "Run
  simulation" button stays as an immediate trigger.
- "Save as scenario" still works — preserves the scenario params but
  not yet the life events / flex rules (a Wave-3 follow-up could
  POST them after the scenario is created).

247 pytest pass; mypy + ruff + frontend typecheck/test/build all clean.

fire_planner/api/schemas.py

@@ -427,11 +427,26 @@ class SpendingProfilePoint(BaseModel):
 
 
 class SpendingProfileResponse(BaseModel):
-    scenario_id: int
+    scenario_id: int | None = None
     horizon_years: int
     points: list[SpendingProfilePoint]
 
 
+class SpendingProfilePreviewRequest(BaseModel):
+    """Stateless spending-profile preview — used by the What-If page where
+    the user is editing in-memory life events that aren't persisted yet.
+
+    No scenario_id needed; the caller supplies the baseline spending,
+    horizon, and any flex rules. Flex-cut estimation against a portfolio
+    fan is skipped (caller already has a /simulate response to pair
+    with this if they want flex visualisation).
+    """
+    base_spending_gbp: Decimal = Field(ge=0)
+    horizon_years: int = Field(ge=1, le=100)
+    life_events: list[LifeEventInput] = Field(default_factory=list)
+    flex_rules: list[FlexRule] = Field(default_factory=list)
+
+
 class SimulateRequest(BaseModel):
     """Sync, non-persisted simulate. Used by the React UI for what-if.
 

fire_planner/api/spending_profile.py

@@ -31,6 +31,7 @@ from sqlalchemy.ext.asyncio import AsyncSession
 from fire_planner.api.dependencies import get_session
 from fire_planner.api.schemas import (
     SpendingProfilePoint,
+    SpendingProfilePreviewRequest,
     SpendingProfileResponse,
 )
 from fire_planner.db import LifeEvent, McRun, ProjectionYearly, Scenario
@@ -183,3 +184,73 @@ async def get_spending_profile(
         horizon_years=horizon,
         points=points,
     )
+
+
+@router.post("/spending-profile-preview", response_model=SpendingProfileResponse)
+async def preview_spending_profile(
+    req: SpendingProfilePreviewRequest,
+) -> SpendingProfileResponse:
+    """Stateless variant — used by the What-If page where the user is
+    tweaking in-memory life events that aren't persisted yet."""
+    points: list[SpendingProfilePoint] = []
+    base = Decimal(str(req.base_spending_gbp))
+    # Flex rules are accepted for forward-compat but not applied here:
+    # without a portfolio path we can't decide drawdown depth. The
+    # paired /simulate call gives the exact cut; this endpoint just
+    # gives the pre-cut profile for the stacked-area chart.
+    _ = req.flex_rules
+
+    for year_idx in range(req.horizon_years):
+        essential = Decimal("0")
+        discretionary = Decimal("0")
+        not_spending = Decimal("0")
+        ess_inflow = Decimal("0")
+        disc_inflow = Decimal("0")
+        for ev in req.life_events:
+            if not ev.enabled:
+                continue
+            if year_idx < ev.year_start:
+                continue
+            end = ev.year_end if ev.year_end is not None else ev.year_start
+            if year_idx > end:
+                continue
+            delta = Decimal(str(ev.delta_gbp_per_year or 0))
+            if ev.category == "not_spending":
+                not_spending += abs(delta)
+                continue
+            if delta < 0:
+                if ev.category == "essential":
+                    essential += -delta
+                elif ev.category == "discretionary":
+                    discretionary += -delta
+            elif delta > 0:
+                if ev.category == "essential":
+                    ess_inflow += delta
+                elif ev.category == "discretionary":
+                    disc_inflow += delta
+            if year_idx == ev.year_start and ev.one_time_amount_gbp is not None:
+                ot = Decimal(str(ev.one_time_amount_gbp))
+                if ot < 0:
+                    if ev.category == "essential":
+                        essential += -ot
+                    elif ev.category == "discretionary":
+                        discretionary += -ot
+        net_base = base - ess_inflow - disc_inflow
+        if net_base < 0:
+            net_base = Decimal("0")
+        total = net_base + essential + discretionary
+        points.append(
+            SpendingProfilePoint(
+                year_idx=year_idx,
+                base_gbp=net_base,
+                essential_gbp=essential,
+                discretionary_gbp=discretionary,
+                not_spending_gbp=not_spending,
+                flex_cut_gbp=Decimal("0"),
+                total_gbp=total,
+            ))
+    return SpendingProfileResponse(
+        scenario_id=None,
+        horizon_years=req.horizon_years,
+        points=points,
+    )