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.