Add multi-currency pricing (USD, CAD, EUR, GBP) for deal posts

Fetches live exchange rates from the Frankfurter API (ECB data, free,
no API key) and displays converted prices alongside USD in every deal
message. Rates are cached in memory and auto-refreshed hourly.

https://claude.ai/code/session_01LPpSZFfyh6vdV5HGFWjoQX

gaming_deals_bot/currency.py

@@ -0,0 +1,96 @@
+"""Currency conversion using the Frankfurter API (ECB rates, no API key required).
+
+Rates are cached in memory and refreshed at most once per hour.
+"""
+
+import logging
+import time
+
+import httpx
+
+logger = logging.getLogger(__name__)
+
+FRANKFURTER_URL = "https://api.frankfurter.dev/v1/latest"
+TARGET_CURRENCIES = ("CAD", "EUR", "GBP")
+CACHE_TTL_SECONDS = 3600  # 1 hour
+
+# Module-level cache
+_rates: dict[str, float] = {}
+_last_fetched: float = 0.0
+
+
+async def refresh_rates(client: httpx.AsyncClient) -> bool:
+    """Fetch latest USD-based exchange rates from Frankfurter.
+
+    Returns True if rates were successfully updated.
+    """
+    global _rates, _last_fetched
+
+    symbols = ",".join(TARGET_CURRENCIES)
+    try:
+        resp = await client.get(
+            FRANKFURTER_URL,
+            params={"base": "USD", "symbols": symbols},
+        )
+        resp.raise_for_status()
+        data = resp.json()
+    except (httpx.HTTPError, ValueError) as exc:
+        logger.warning("Failed to fetch exchange rates: %s", exc)
+        return False
+
+    new_rates = data.get("rates", {})
+    if not new_rates:
+        logger.warning("Frankfurter returned empty rates")
+        return False
+
+    _rates = {k: float(v) for k, v in new_rates.items()}
+    _last_fetched = time.monotonic()
+    logger.info("Exchange rates updated: %s", _rates)
+    return True
+
+
+async def _ensure_rates(client: httpx.AsyncClient) -> None:
+    """Refresh rates if the cache is stale or empty."""
+    if not _rates or (time.monotonic() - _last_fetched) > CACHE_TTL_SECONDS:
+        await refresh_rates(client)
+
+
+def _convert(usd_amount: float, currency: str) -> float | None:
+    """Convert a USD amount to the target currency using cached rates."""
+    rate = _rates.get(currency)
+    if rate is None:
+        return None
+    return round(usd_amount * rate, 2)
+
+
+# Currency display symbols
+_SYMBOLS = {
+    "USD": "$",
+    "CAD": "C$",
+    "EUR": "€",
+    "GBP": "£",
+}
+
+
+def format_price(usd_amount: float) -> str:
+    """Return a formatted multi-currency price string.
+
+    Example: ``$14.99 · C$20.54 · €13.78 · £11.98``
+
+    Falls back to USD-only if rates aren't available.
+    """
+    parts = [f"${usd_amount:.2f}"]
+
+    for cur in TARGET_CURRENCIES:
+        converted = _convert(usd_amount, cur)
+        if converted is not None:
+            symbol = _SYMBOLS.get(cur, cur)
+            parts.append(f"{symbol}{converted:.2f}")
+
+    return " · ".join(parts)
+
+
+async def format_price_async(client: httpx.AsyncClient, usd_amount: float) -> str:
+    """Ensure rates are fresh, then format a multi-currency price string."""
+    await _ensure_rates(client)
+    return format_price(usd_amount)