Context
-------
FX for UK users has two lives: live ECB rates for portfolio display
(available same-day), and HMRC monthly/daily rates for CGT basis
(published after month-end). The plan keeps both in one cache table
with an upgradable `source` column, so a later reconciliation job can
replace ECB_LIVE values with HMRC_MONTHLY for the same date without
schema work.
This change
-----------
- FxCache: SQLite table (currency, on_date) -> (rate_gbp, source) with
ON CONFLICT UPDATE semantics so reconciliation is a single put().
- convert_to_gbp(): GBP short-circuits to identity; any other currency
must be in the cache (network fetch is the caller's responsibility,
separately implemented by the ECB and HMRC fetchers).
- Explicit LookupError on cache miss — deliberate, we do NOT want a
silent fallback that produces wrong cost-basis numbers.
Decisions deferred to later commits:
- Actual ECB daily reference-rate fetcher (eurofxref XML) — lands with
the Trading212 provider in Phase 1 when non-GBP trades first appear.
- HMRC monthly-rate fetcher + reconciliation CronJob — Phase 1 tail.
Test plan
---------
## Automated
- poetry run pytest tests/test_fx.py -v → 6 passed
- poetry run mypy broker_sync tests → Success: no issues found in 8 source files
- poetry run ruff check . → All checks passed!
## Manual Verification
Not applicable — no network yet.
