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

config.json

@@ -0,0 +1,14 @@
+{
+  "routes": [
+    {
+      "origin": "BER",
+      "destination": "EZE",
+      "trip_type": "round_trip",
+      "threshold_eur": 700,
+      "duration_weeks": [2, 3, 4],
+      "preferred_months": [12, 1, 2, 3],
+      "monitor_year_round": true
+    }
+  ],
+  "check_interval": "daily"
+}