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.

alembic/versions/0004_life_event_category.py

@@ -0,0 +1,38 @@
+"""add life_event.category for flex-spending classification
+
+Revision ID: 0004
+Revises: 0003
+Create Date: 2026-05-10 14:00:00.000000
+
+Wave 2 chart-first redesign — life events tag their spending impact as
+essential / discretionary / not_spending so flex-spending rules can
+trim discretionary outflows when the portfolio drops below ATH.
+"""
+from collections.abc import Sequence
+
+import sqlalchemy as sa
+
+from alembic import op
+
+revision: str = "0004"
+down_revision: str | None = "0003"
+branch_labels: str | Sequence[str] | None = None
+depends_on: str | Sequence[str] | None = None
+
+SCHEMA = "fire_planner"
+
+
+def upgrade() -> None:
+    with op.batch_alter_table("life_event", schema=SCHEMA) as batch:
+        batch.add_column(
+            sa.Column(
+                "category",
+                sa.Text(),
+                nullable=False,
+                server_default=sa.text("'essential'"),
+            ))
+
+
+def downgrade() -> None:
+    with op.batch_alter_table("life_event", schema=SCHEMA) as batch:
+        batch.drop_column("category")