Convert USD and EUR to CRC in analytics endpoints

All three analytics endpoints (by-category, monthly-trend, daily-spending)
now convert foreign currency amounts to CRC using current exchange rates.
EUR/CRC rate derived from ExchangeRate-API (USD-based cross rate).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/services/exchange_rate.py

@@ -23,6 +23,10 @@ _cache: dict[str, tuple[ExchangeRate, datetime]] = {}
 _last_known: ExchangeRate | None = None  # survives cache expiry — always holds the last successful rate
 CACHE_TTL = timedelta(hours=1)
 
+# EUR/CRC mid-market rate cache
+_eur_crc_cache: dict[str, tuple[float, datetime]] = {}
+_last_known_eur_crc: float | None = None
+
 
 def _fetch_bccr_rate(indicator: int, date_str: str) -> float | None:
     """Fetch a single indicator from BCCR API."""
@@ -167,6 +171,45 @@ def get_current_rate(session: Session) -> ExchangeRate | None:
     return None
 
 
+def _fetch_eur_crc_mid() -> float | None:
+    """Derive EUR/CRC mid-market rate from ExchangeRate-API (USD-based).
+
+    EUR/CRC = CRC_per_USD / EUR_per_USD
+    """
+    try:
+        resp = httpx.get(EXCHANGERATE_API_URL, timeout=10)
+        resp.raise_for_status()
+        data = resp.json()
+        if data.get("result") == "success":
+            crc = data["rates"].get("CRC")
+            eur = data["rates"].get("EUR")
+            if crc and eur:
+                return float(crc) / float(eur)
+    except Exception:
+        pass
+    return None
+
+
+def get_eur_crc_rate() -> float | None:
+    """Get current EUR→CRC mid-market rate (cached 1 hour)."""
+    global _last_known_eur_crc
+
+    cached = _eur_crc_cache.get("current")
+    if cached and datetime.utcnow() - cached[1] < CACHE_TTL:
+        return cached[0]
+
+    rate = _fetch_eur_crc_mid()
+    if rate is not None:
+        _eur_crc_cache["current"] = (rate, datetime.utcnow())
+        _last_known_eur_crc = rate
+        return rate
+
+    if _last_known_eur_crc:
+        return _last_known_eur_crc
+
+    return None
+
+
 def get_rate_history(session: Session, days: int = 30) -> list[ExchangeRate]:
     """Get historical exchange rates."""
     cutoff = datetime.utcnow() - timedelta(days=days)