Stazioni: camera_url e camera_source, GET /api/stations/by-code/{code} (#72)

Migrazione 020 con le due colonne su stations, campi in modello, schemi e
servizio (validatore http(s) come vision_worker_url), nuova route by-code
dichiarata prima di /{station_id}. Test di migrazione su e giu e di API.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/backend/tests/test_migrations.py

@@ -321,3 +321,23 @@ class TestStationVisionWorkerMigration:
         command.upgrade(cfg, "head")  # e di nuovo su
         assert "vision_worker_url" in cols("stations")
         assert "worker_url" in cols("vision_results")
+
+
+class TestStationCameraMigration:
+    """020_station_camera: stations.camera_url e stations.camera_source, su e giu (#72)."""
+
+    def test_020_upgrade_and_downgrade(self, sqlite_database_url):
+        db_path, _ = sqlite_database_url
+        cfg = _alembic_config()
+
+        def cols(table):
+            return {c["name"] for c in _inspect(db_path).get_columns(table)}
+
+        command.upgrade(cfg, "head")
+        assert {"camera_url", "camera_source"} <= cols("stations")
+        command.downgrade(cfg, "019_station_vision_worker")
+        assert "camera_url" not in cols("stations")
+        assert "camera_source" not in cols("stations")
+        assert "vision_worker_url" in cols("stations")  # la 019 resta
+        command.upgrade(cfg, "head")  # e di nuovo su
+        assert {"camera_url", "camera_source"} <= cols("stations")