feat: Initial project scaffold

Flask API backend for ESP32 sensor fleet:
- App factory pattern with blueprints
- SQLAlchemy 2.x models (Sensor, Device, Sighting, Alert, Event, Probe)
- UDP collector for sensor data streams
- REST API endpoints for sensors, devices, alerts, events, probes, stats
- pytest setup with fixtures
- Containerfile for podman deployment
- Makefile for common tasks

.env.example

@@ -0,0 +1,11 @@
+# Flask
+SECRET_KEY=change-me-in-production
+FLASK_ENV=development
+
+# Database
+DATABASE_URL=sqlite:///esp32.db
+
+# Network
+UDP_PORT=5500
+CMD_PORT=5501
+SENSOR_TIMEOUT=60