FASE 4-5-6: Infraestructura, CRM, Service Orders, Notificaciones, Ahorro, Logistica, API Publica

FASE 4:
- Redis cache de stock con fallback graceful
- Multi-moneda (MXN/USD) con contabilidad en MXN
- Proveedores y ordenes de compra completo
- Meilisearch 1.5M+ partes indexadas
- Metabase KPIs con dashboard auto-generado

FASE 5:
- CRM mejorado: activities, tags, loyalty program, analytics
- Imagenes de partes: upload, resize, thumbnails WebP
- Ordenes de servicio Kanban: received->diagnosis->repair->ready->delivered
- Garantias/RMA, alertas de reorden, multi-sucursal
- Stubs BNPL (APLAZO) y ERP Sync (Aspel/Contpaqi)

FASE 6:
- Notificaciones automaticas: push/WhatsApp/email/in-app
- Reportes de ahorro vs retail_price
- Logistica + tracking: DHL, FedEx, Estafeta, 99min, Uber
- API Publica: API keys, rate limiting, catalog search

Migraciones: v1.9-v3.0
Tests: 93/93 pasando
Backup: nexus_backup_20260427_045859.tar.gz

pos/services/currency.py

@@ -1,48 +1,131 @@
 """Multi-currency support for border refaccionarias.
 
-Supports MXN and USD with configurable exchange rate.
+Supports MXN and USD with configurable exchange rate per tenant.
+Rates are cached in Redis for 60 seconds to avoid repeated DB hits.
+
+Business rule: inventory prices are ALWAYS in MXN (base currency).
+Sales can be recorded in USD with conversion at checkout time.
+Accounting and CFDI always use MXN.
 """
 
+from decimal import Decimal, ROUND_HALF_UP
+
 from config import DEFAULT_CURRENCY, EXCHANGE_RATE_USD_MXN
+from services.redis_stock_cache import _get_redis
 
 CURRENCIES = {
     'MXN': {'symbol': '$', 'name': 'Peso Mexicano', 'name_en': 'Mexican Peso', 'decimals': 2},
     'USD': {'symbol': 'US$', 'name': 'Dolar Estadounidense', 'name_en': 'US Dollar', 'decimals': 2},
 }
 
+# Cache TTL for exchange rates in Redis (seconds)
+_RATE_TTL = 60
 
-def convert(amount, from_currency, to_currency, rate=None):
+
+def _to_dec(val):
+    if val is None:
+        return Decimal('0')
+    return Decimal(str(val))
+
+
+def get_exchange_rate(conn, from_currency, to_currency):
+    """Get the exchange rate from tenant_config, with Redis cache.
+
+    Returns:
+        Decimal: rate such that amount * rate = converted amount
+        (e.g., USD->MXN returns ~17.5, MXN->USD returns ~0.057)
+    """
+    if from_currency == to_currency:
+        return Decimal('1')
+
+    cache_key = f"nexus:rate:{from_currency}:{to_currency}"
+
+    # Try Redis first
+    r = _get_redis()
+    if r:
+        try:
+            cached = r.get(cache_key)
+            if cached:
+                return Decimal(str(cached))
+        except Exception:
+            pass
+
+    # Fallback: read from tenant_config DB
+    rate = None
+    if conn:
+        try:
+            cur = conn.cursor()
+            cur.execute(
+                "SELECT value FROM tenant_config WHERE key = 'exchange_rate_usd_mxn'"
+            )
+            row = cur.fetchone()
+            cur.close()
+            if row and row[0]:
+                rate = Decimal(str(row[0]))
+        except Exception:
+            pass
+
+    if rate is None:
+        rate = _to_dec(EXCHANGE_RATE_USD_MXN)
+
+    # Compute cross rate
+    if from_currency == 'USD' and to_currency == 'MXN':
+        result = rate
+    elif from_currency == 'MXN' and to_currency == 'USD':
+        result = (Decimal('1') / rate).quantize(Decimal('0.000001'), rounding=ROUND_HALF_UP)
+    else:
+        result = Decimal('1')
+
+    # Cache in Redis
+    if r:
+        try:
+            r.set(cache_key, str(result), ex=_RATE_TTL)
+        except Exception:
+            pass
+
+    return result
+
+
+def convert(amount, from_currency, to_currency, rate=None, conn=None):
     """Convert an amount between currencies.
 
     Args:
-        amount: The numeric amount to convert.
-        from_currency: Source currency code ('MXN' or 'USD').
-        to_currency: Target currency code ('MXN' or 'USD').
-        rate: Optional custom exchange rate (USD->MXN). Defaults to config value.
+        amount: numeric amount to convert.
+        from_currency: source currency code.
+        to_currency: target currency code.
+        rate: optional pre-computed rate (skips DB lookup).
+        conn: optional DB connection to look up tenant rate.
 
     Returns:
-        The converted amount, rounded to 2 decimals.
+        float: converted amount rounded to 2 decimals.
     """
     if from_currency == to_currency:
-        return amount
+        return float(amount)
+
     if rate is None:
-        rate = EXCHANGE_RATE_USD_MXN
-    if from_currency == 'USD' and to_currency == 'MXN':
-        return round(amount * rate, 2)
-    if from_currency == 'MXN' and to_currency == 'USD':
-        return round(amount / rate, 2)
-    return amount
+        rate = get_exchange_rate(conn, from_currency, to_currency)
+
+    amt = _to_dec(amount)
+    rate_dec = _to_dec(rate)
+    result = (amt * rate_dec).quantize(Decimal('0.01'), rounding=ROUND_HALF_UP)
+    return float(result)
+
+
+def to_mxn(amount, currency, rate=None, conn=None):
+    """Convert an amount to MXN (convenience wrapper)."""
+    return convert(amount, currency, 'MXN', rate=rate, conn=conn)
+
+
+def from_mxn(amount, currency, rate=None, conn=None):
+    """Convert an amount from MXN to target currency."""
+    return convert(amount, 'MXN', currency, rate=rate, conn=conn)
 
 
 def format_currency(amount, currency='MXN'):
     """Format an amount with the appropriate currency symbol.
 
-    Args:
-        amount: Numeric value.
-        currency: Currency code.
-
     Returns:
-        Formatted string like '$1,234.56' or 'US$1,234.56'.
+        str: e.g. '$1,234.56' or 'US$1,234.56'.
     """
     info = CURRENCIES.get(currency, CURRENCIES['MXN'])
     return f"{info['symbol']}{amount:,.{info['decimals']}f}"
@@ -52,4 +135,17 @@ def get_currency_info(code=None):
     """Return currency metadata dict. If code is None, return all."""
     if code:
         return CURRENCIES.get(code)
-    return CURRENCIES
+    return CURRENCIES.copy()
+
+
+def invalidate_rate_cache():
+    """Clear all cached exchange rates from Redis."""
+    r = _get_redis()
+    if not r:
+        return
+    try:
+        keys = r.keys('nexus:rate:*')
+        if keys:
+            r.delete(*keys)
+    except Exception:
+        pass