feat: initial flight monitor

- Python flight price monitor for BER↔EZE and other routes
- Adapter pattern for extensibility (Kiwi/Tequila implemented)
- OpenClaw alerting integration
- Cron-friendly one-shot execution
- Full logging of all checks
- Graceful handling when API key not set

Implements monkey-island/flight-monitor#2

adapters/base.py

@@ -0,0 +1,54 @@
+"""
+Base adapter interface for flight price sources.
+"""
+from abc import ABC, abstractmethod
+from datetime import date
+from typing import List, Dict, Any
+
+
+class FlightAdapter(ABC):
+    """
+    Abstract base class for flight price adapters.
+    
+    Each adapter implements a specific price source (Kiwi, Google Flights, etc.)
+    and returns results in a normalized format.
+    """
+    
+    @abstractmethod
+    def search_round_trip(
+        self,
+        origin: str,
+        destination: str,
+        departure_date: date,
+        return_date: date,
+        **kwargs
+    ) -> List[Dict[str, Any]]:
+        """
+        Search for round-trip flights.
+        
+        Args:
+            origin: IATA code of origin airport (e.g., "BER")
+            destination: IATA code of destination airport (e.g., "EZE")
+            departure_date: Departure date
+            return_date: Return date
+            **kwargs: Adapter-specific parameters
+        
+        Returns:
+            List of results, each with:
+            {
+                "price_eur": float,
+                "airline": str,
+                "departure": datetime,
+                "return": datetime,
+                "link": str (booking URL),
+                "source": str (adapter name)
+            }
+            
+            Returns empty list if no results found.
+        """
+        pass
+    
+    @abstractmethod
+    def get_name(self) -> str:
+        """Return the name of this adapter."""
+        pass