fix: restore missing API functions in mercado_publico.py to resolve backend startup error

backend/app/services/mercado_publico.py

@@ -170,6 +170,20 @@ async def _fetch(params: Dict[str, str], retries: int = 3) -> List[Tender]:
                     return []
     return []
 
+async def get_active_tenders() -> List[Tender]:
+    """Fetch tenders active today."""
+    return await _fetch({})
+
+async def get_tenders_by_date(date_ddmmaaaa: str) -> List[Tender]:
+    """Fetch tenders for a specific date (ddmmaaaa)."""
+    return await _fetch({"fecha": date_ddmmaaaa})
+
+async def get_tender_by_code(code: str) -> Optional[Tender]:
+    """Fetch a single tender by its external code."""
+    tenders = await _fetch({"codigo": code})
+    return tenders[0] if tenders else None
+
+
 async def get_tenders_by_filters(
     date: Optional[str] = None,
     status: Optional[str] = None,