examples: to_gbp currency normalisation helper

fire_planner/examples/llm_extract.py

@@ -179,3 +179,22 @@ async def extract_with_fallback(
         client=client,
     )
     return secondary or primary
+
+
+def to_gbp(
+    amount: Decimal | None,
+    currency: str | None,
+    rates: dict[str, Decimal],
+) -> Decimal | None:
+    """Convert `amount` in `currency` to GBP using `fx.fetch_rates` output.
+
+    `rates[X]` = "how much GBP one unit of X is worth" — the convention
+    used by `fire_planner/fx.py`. Returns None when amount/currency is
+    missing or the currency isn't in `rates`.
+    """
+    if amount is None or currency is None:
+        return None
+    rate = rates.get(currency.upper())
+    if rate is None:
+        return None
+    return (amount * rate).quantize(Decimal("0.01"))