feat: flight price monitor with Kiwi adapter stub and cron setup

- config.json: BER→EZE route, €700 threshold, 14-28 nights, Dec-Mar preferred
- monitor.py: config-driven, multi-route, windowed date search, openclaw alerts
- adapters/base.py: abstract FlightAdapter interface
- adapters/kiwi.py: Kiwi/Tequila v2 adapter (stub until KIWI_API_KEY is set)
- requirements.txt, cron-install.sh (daily 08:00 UTC), README.md

adapters/base.py

@@ -0,0 +1,42 @@
+"""Base adapter interface for flight price sources."""
+
+from abc import ABC, abstractmethod
+from typing import Optional
+
+
+class FlightAdapter(ABC):
+    """Abstract base class for flight search adapters.
+
+    All adapters must return a list of flight result dicts with keys:
+        price_eur   (float)  - total round-trip price in EUR
+        outbound_date (str)  - departure date YYYY-MM-DD
+        return_date   (str)  - return date YYYY-MM-DD
+        deep_link     (str)  - booking URL
+        carrier       (str)  - airline name or IATA code
+    """
+
+    @abstractmethod
+    def search(
+        self,
+        origin: str,
+        destination: str,
+        date_from: str,
+        date_to: str,
+        min_nights: int,
+        max_nights: int,
+    ) -> list[dict]:
+        """Search for round-trip flights.
+
+        Args:
+            origin:      IATA airport/city code (e.g. "BER")
+            destination: IATA airport/city code (e.g. "EZE")
+            date_from:   Earliest outbound date, YYYY-MM-DD
+            date_to:     Latest outbound date, YYYY-MM-DD
+            min_nights:  Minimum stay duration
+            max_nights:  Maximum stay duration
+
+        Returns:
+            List of result dicts (may be empty). Sorted by price ascending
+            is preferred but not required.
+        """
+        ...