fire-planner: Wave 2 chart-first — flex spending, categorised life

events, interactive Visx Gantt + spending-profile chart

Charts are now the primary editor for life events. The Plan-tab body
re-orders to make charts ~80% of viewport real-estate; legacy form
sections are collapsed into a drawer.

Backend:
- alembic 0004: life_event.category enum (essential / discretionary /
  not_spending). Defaults to essential so existing rows keep their
  full spending impact.
- Simulator gains discretionary_outflows + flex_rules params. Tracks
  per-path running ATH, applies the deepest applicable cut to
  discretionary outflows when portfolio drops vs ATH (PLab-style flex
  spending). Cut amount stays in the portfolio (refund pattern).
- New flex_spending module with FlexRule + applicable_cut +
  cuts_per_year (vectorised). Sortable rules; "deepest cut wins" so
  users specify cumulative cuts at each tier.
- New /scenarios/{id}/spending-profile endpoint returning per-year
  base / essential / discretionary / flex_cut / total breakdown.
- SimulateRequest gains flex_rules + life_event.category roundtrip.
- 8 new tests; 246 total pytest pass; mypy + ruff clean.

Frontend (Visx + ECharts):
- Installed @visx/{scale,shape,group,axis,event,responsive,tooltip}
  for native SVG drag interactions.
- New <SpendingProfileChart> — Visx stacked-area of base/essential/
  discretionary with red flex-cut overlay, hover tooltip, click-to-
  scrub-year.
- New <EventGantt> — interactive Visx Gantt:
    * Click empty space → popover create at that year (default
      essential spending event)
    * Click a bar → inline edit popover (name, kind, range, £/y,
      category) with delete button
    * Drag bar middle → moves the whole event (year-resolution snap)
    * Drag bar edges → resizes year_start / year_end
    * All gestures persist via PATCH /life-events/{id}
- New <FlexRulesEditor> — list of {from_ath_pct, cut} tiers, save-on-
  change to scenario.config_json.flex_rules.
- Plan-tab redesign: NW fan dominant top with floating stat badges
  (Year/Age/NW/Δ NW/Spending/Eff. tax) over the chart; spending-
  profile chart middle; Gantt bottom; flex-rules editor; legacy form
  sections in a collapsed <details> drawer.
- Frontend typecheck + 7 vitest tests + production build all clean.

fire_planner/db.py

@@ -192,6 +192,15 @@ class LifeEvent(Base):
                                                         nullable=False,
                                                         server_default=text("0"))
     one_time_amount_gbp: Mapped[Decimal | None] = mapped_column(Numeric(14, 2), nullable=True)
+    # Spending category for flex-spending classification:
+    #   essential       — never trimmed by flex rules (mortgage, food, kids)
+    #   discretionary   — trimmed when portfolio drops vs ATH (travel, dining)
+    #   not_spending    — informational only (a milestone marker that doesn't
+    #                     change cashflow, e.g. a kid graduating)
+    # Default is `essential` so existing rows keep their full spending impact.
+    category: Mapped[str] = mapped_column(String(16),
+                                          nullable=False,
+                                          server_default=text("'essential'"))
     enabled: Mapped[bool] = mapped_column(Boolean, nullable=False, server_default=text("true"))
     payload: Mapped[dict[str, Any] | None] = mapped_column(JSON_TYPE, nullable=True)
     created_at: Mapped[datetime] = mapped_column(TIMESTAMP(timezone=True),