Add ECB FX fetcher + cache population

Context
-------
Phase 1 needs live FX rates for USD-denominated RSU vestings (Schwab),
EUR-denominated deposits, and any multi-currency dividend. The FxCache
from an earlier commit stores (currency, date) → rate_to_gbp but was
intentionally left empty — this commit wires the ingestion path.

Design decisions
----------------
- ECB publishes EUR→X, not X→GBP. Everything pivots through EUR:
      rate(X→GBP) = rate(EUR→GBP) / rate(EUR→X)
  GBP goes into the result at 1.0 so callers iterating the dict get
  a consistent shape; `populate_fx_cache` then skips GBP because
  `convert_to_gbp` has a dedicated passthrough branch.
- `on_date` parameter is accepted for API symmetry with the future
  historical fetcher even though the daily endpoint only serves the
  most recent publication. The docstring calls this out explicitly.
- XML is parsed with stdlib `xml.etree.ElementTree`. No `lxml` —
  the file is 30 lines, no performance concern, and keeping deps
  minimal matters for the container image.
- The HTTP layer takes an optional `httpx.AsyncBaseTransport` the same
  way WealthfolioSink does — MockTransport drives all tests, the
  production caller just leaves it None.

This change
-----------
- broker_sync/fx_ecb.py:
  * `fetch_ecb_rates(on_date, *, transport=None)` — GETs the daily
    XML, parses, pivots through EUR, returns `{ccy: rate_to_gbp}`.
    Raises `RuntimeError` on non-2xx or if GBP is missing (cannot
    pivot). No retries — caller handles resilience.
  * `populate_fx_cache(cache, rates, on_date)` — writes every
    non-GBP rate with `FxRateSource.ECB_LIVE`.
  * `fetch_ecb_rates_historical(start, end)` — `NotImplementedError`
    stub; filed as beads task code-thw.2.2. Needed for backfilling
    years of T212 history (daily endpoint only covers today).
- tests/fixtures/ecb_2026-04-01.xml: realistic 5-currency ECB snapshot.
- tests/test_fx_ecb.py: fixture-driven tests covering the pivot math,
  the 503 failure path, the cache-skip-GBP rule, and the NotImplemented
  guard on the historical stub.

Test plan
---------
## Automated
- poetry run pytest -q  →  52 passed in 0.50s
- poetry run mypy broker_sync tests  →  Success: no issues found in 26 source files
- poetry run ruff check .  →  All checks passed!

## Manual Verification
Live daily endpoint hit deferred to the CLI integration commit — the
fetcher is pure + transport-injectable, so the unit tests cover the
parsing and pivot logic, and the CLI wiring will be the place where
the live call is exercised end-to-end.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

broker_sync/fx_ecb.py

@@ -0,0 +1,100 @@
+"""ECB daily reference-rate fetcher.
+
+The ECB publishes today's rates as an XML document at
+https://www.ecb.europa.eu/stats/eurofxref/eurofxref-daily.xml. Rates are
+given as EUR→X, so deriving rate(X→GBP) needs a pivot through EUR:
+
+    rate(X→GBP) = rate(EUR→GBP) / rate(EUR→X)
+
+The daily file only contains the most recent publication — historical
+backfill needs the 90-day (`eurofxref-hist-90d.xml`) or full-history
+(`eurofxref-hist.xml`) endpoints. Those are not wired yet because
+backfilling years of T212 history is a Phase 1.1 concern; for now
+`fetch_ecb_rates_historical` raises NotImplementedError.
+"""
+from __future__ import annotations
+
+from datetime import date
+from decimal import Decimal
+from xml.etree import ElementTree as ET
+
+import httpx
+
+from broker_sync.fx import FxCache
+from broker_sync.models import FxRateSource
+
+_ECB_DAILY_URL = "https://www.ecb.europa.eu/stats/eurofxref/eurofxref-daily.xml"
+_ECB_NS = "{http://www.ecb.int/vocabulary/2002-08-01/eurofxref}"
+
+
+async def fetch_ecb_rates(
+    on_date: date,
+    *,
+    transport: httpx.AsyncBaseTransport | None = None,
+) -> dict[str, Decimal]:
+    """Return `{currency: rate_to_gbp}` for every currency the ECB publishes.
+
+    GBP is present in the result as rate 1.0 (the pivot target). EUR is
+    present with its direct EUR→GBP rate. Everything else is pivoted
+    through EUR.
+
+    `on_date` is accepted for API symmetry with the historical fetcher
+    but the daily endpoint always returns the latest publication — the
+    caller is responsible for only invoking this for today's date.
+    """
+    del on_date
+    async with httpx.AsyncClient(transport=transport, timeout=30.0) as c:
+        resp = await c.get(_ECB_DAILY_URL)
+    if resp.status_code >= 400:
+        raise RuntimeError(f"ECB daily fetch failed: HTTP {resp.status_code}")
+
+    eur_rates = _parse_eur_rates(resp.text)
+    if "GBP" not in eur_rates:
+        raise RuntimeError("ECB response missing GBP — cannot pivot")
+
+    eur_to_gbp = eur_rates["GBP"]
+    out: dict[str, Decimal] = {"GBP": Decimal("1"), "EUR": eur_to_gbp}
+    for ccy, eur_to_ccy in eur_rates.items():
+        if ccy == "GBP":
+            continue
+        out[ccy] = eur_to_gbp / eur_to_ccy
+    return out
+
+
+def _parse_eur_rates(xml_text: str) -> dict[str, Decimal]:
+    root = ET.fromstring(xml_text)
+    rates: dict[str, Decimal] = {}
+    for cube in root.iter(f"{_ECB_NS}Cube"):
+        ccy = cube.get("currency")
+        rate = cube.get("rate")
+        if ccy and rate:
+            rates[ccy] = Decimal(rate)
+    return rates
+
+
+async def populate_fx_cache(
+    cache: FxCache,
+    rates: dict[str, Decimal],
+    on_date: date,
+) -> None:
+    """Write non-GBP rates into the FxCache tagged `ECB_LIVE`.
+
+    GBP is intentionally skipped — `convert_to_gbp` short-circuits on
+    GBP without touching the cache.
+    """
+    for ccy, rate in rates.items():
+        if ccy == "GBP":
+            continue
+        cache.put(ccy, on_date, rate, FxRateSource.ECB_LIVE)
+
+
+async def fetch_ecb_rates_historical(
+    start: date,
+    end: date,
+) -> dict[date, dict[str, Decimal]]:
+    """Placeholder for the 90-day / full-history ECB endpoints.
+
+    Tracked by beads task: see parent Phase 1 epic.
+    """
+    raise NotImplementedError("Historical ECB fetch not wired; needs eurofxref-hist-90d.xml or "
+                              "eurofxref-hist.xml — filed as a follow-up beads task.")