feat(fire-target): per-Case FIRE-number solver for the retirement countdown

Add a Monte-Carlo "FIRE number" solver so the wealth dashboard can show a £
countdown to retirement across life-stage cases, in today's money.

Viktor wants to see, per country, how far his net worth is from being able to
retire for good under three cases — Solo (his spend ×1.5), Household (+Anca
×1.5), Family (+2 kids) — with cost-of-living re-scaling per country and a 99%
Guyton-Klinger success bar.

- spend_model: per-Case real-GBP spend, COL-scaled (rent + non-rent essentials
  scale by country; Holidays fixed), ×1.5 safety. Constants sourced live from
  actualbudget (Viktor) / on-record (Anca).
- geo: city -> tax jurisdiction (nomad fallback).
- fire_target: binary-search the smallest LIQUID net worth where GK reaches the
  bar; pension modelled as a tranche unlocking at ~57, kids ramp + optional home
  as cashflows. New fire_target table (migration 0007) + idempotent upsert.
- recompute-fire-targets CLI: solve every Case x country and persist for Grafana.
- CONTEXT.md glossary + ADR-0001 (why MC-threshold on liquid NW, not 25x spend).

Reuses the existing simulator unchanged (its cashflow hooks already supported
pension/kids/home). 345 tests pass; mypy + ruff clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

CONTEXT.md

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+# FIRE Planner — Context
+
+A personal FIRE engine: a Monte-Carlo retirement-success simulator plus a live
+"can I retire yet?" countdown that tracks net worth against per-life-stage targets.
+
+## Language
+
+**Case**:
+A named life-stage the countdown tracks. Solo (Viktor only), Household (Viktor +
+Anca), Family (Household + 2 kids). Each Case is a real-GBP annual spend.
+_Avoid_: scenario (reserved for the Cartesian simulator row — jurisdiction ×
+strategy × leave-year × glide).
+
+**FIRE number / Target NW**:
+The smallest liquid net worth (real GBP, country-specific) at which a Case's
+Guyton-Klinger plan reaches the bar. Computed by Monte-Carlo threshold, not a
+fixed SWR multiple.
+_Avoid_: goal, magic number, the number.
+
+**The bar**:
+The Monte-Carlo success probability that counts as "can retire" — 99%.
+
+**Countdown**:
+Target NW − current liquid NW, in real GBP (today's money). Rendered with
+progress %, projected date, and runway.
+
+**Bridge pot / liquid NW**:
+Assets spendable before the workplace pension unlocks (~57). Funds the early
+sequence-risk years. The pension is excluded from it and modelled as a tranche
+that joins later.
+_Avoid_: net worth (which includes the locked pension).
+
+**COL-driven spend**:
+Spend that re-scales by a country's cost of living — rent (by 1-bed rent ratio)
+and non-rent essentials (by the no-rent basket ratio), plus kids' costs.
+_Contrast_: **Fixed spend** — Holidays, globally priced, unchanged by country.
+
+**Safety multiplier**:
+The ×1.5 padding Viktor applies on top of measured real spend.
+
+**Re-entry trigger**:
+The written rule — take paid work if the portfolio is below £1.0M for two
+consecutive quarters. A guardrail, surfaced on the dashboard, not a failure.