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>

tests/test_fire_targets_cli_helpers.py

@@ -0,0 +1,73 @@
+"""DB helpers behind `recompute-fire-targets` — latest-snapshot net worth split
+and COL lookups. Locks the SQL (the WORKPLACE_PENSION filter especially)."""
+from __future__ import annotations
+
+from datetime import UTC, date, datetime
+from decimal import Decimal
+
+import pytest
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from fire_planner.__main__ import (
+    _all_city_slugs,
+    _current_liquid_and_pension,
+    _load_col_latest,
+)
+from fire_planner.db import AccountSnapshot, ColSnapshot
+
+
+def _acct(ext: str, d: date, atype: str, gbp: str) -> AccountSnapshot:
+    return AccountSnapshot(
+        external_id=ext, snapshot_date=d, account_id=ext, account_name=atype,
+        account_type=atype, currency="GBP",
+        market_value=Decimal(gbp), market_value_gbp=Decimal(gbp),
+    )
+
+
+def _col(slug: str, disp: str, d: date, no_rent: str, rent: str) -> ColSnapshot:
+    return ColSnapshot(
+        city_slug=slug, city_display=disp, country=disp, source_name="baseline",
+        snapshot_date=d, expires_at=datetime(2027, 1, 1, tzinfo=UTC),
+        total_no_rent_gbp=Decimal(no_rent), total_with_rent_gbp=Decimal(no_rent),
+        rent_1bed_center_gbp=Decimal(rent),
+    )
+
+
+async def test_liquid_and_pension_use_latest_date_and_split_pension(
+    session: AsyncSession,
+) -> None:
+    # An older snapshot that must be ignored.
+    session.add(_acct("old:isa", date(2026, 1, 1), "ISA", "1.00"))
+    # Latest date: two liquid accounts + one locked pension.
+    session.add_all([
+        _acct("gia", date(2026, 6, 20), "GIA", "761000.00"),
+        _acct("isa", date(2026, 6, 20), "ISA", "231000.00"),
+        _acct("pension", date(2026, 6, 20), "WORKPLACE_PENSION", "139000.00"),
+    ])
+    await session.commit()
+
+    liquid, pension = await _current_liquid_and_pension(session)
+    assert liquid == pytest.approx(992_000.0)
+    assert pension == pytest.approx(139_000.0)
+
+
+async def test_load_col_latest_picks_most_recent(session: AsyncSession) -> None:
+    session.add_all([
+        _col("sofia", "Sofia", date(2025, 1, 1), "600", "500"),
+        _col("sofia", "Sofia", date(2026, 5, 20), "713", "679"),
+    ])
+    await session.commit()
+    row = await _load_col_latest(session, "sofia")
+    assert row is not None
+    assert row.total_no_rent_gbp == Decimal("713")
+    assert await _load_col_latest(session, "atlantis") is None
+
+
+async def test_all_city_slugs_is_distinct_sorted(session: AsyncSession) -> None:
+    session.add_all([
+        _col("sofia", "Sofia", date(2026, 5, 1), "713", "679"),
+        _col("sofia", "Sofia", date(2026, 5, 20), "713", "679"),
+        _col("lisbon", "Lisbon", date(2026, 5, 1), "900", "1100"),
+    ])
+    await session.commit()
+    assert await _all_city_slugs(session) == ["lisbon", "sofia"]