Add OSM Track Harvesting Policy and demo database initialization script

- Updated documentation to include OSM Track Harvesting Policy with details on railway types, service filters, usage filters, and geometry guardrails.
- Introduced a new script `init_demo_db.py` to automate the database setup process, including environment checks, running migrations, and loading OSM fixtures for demo data.

backend/tests/test_tracks_import.py

@@ -67,3 +67,44 @@ def test_normalize_track_elements_marks_oneway_and_status() -> None:
     track = tracks[0]
     assert track["status"] == "disused"
     assert track["isBidirectional"] is False
+
+
+def test_normalize_track_elements_skips_service_tracks() -> None:
+    elements = [
+        {
+            "type": "way",
+            "id": 77,
+            "geometry": [
+                {"lat": 52.5000, "lon": 13.4000},
+                {"lat": 52.5010, "lon": 13.4010},
+            ],
+            "tags": {
+                "railway": "rail",
+                "service": "yard",
+            },
+        }
+    ]
+
+    tracks = tracks_import.normalize_track_elements(elements)
+
+    assert tracks == []
+
+
+def test_normalize_track_elements_skips_short_tracks() -> None:
+    elements = [
+        {
+            "type": "way",
+            "id": 81,
+            "geometry": [
+                {"lat": 52.500000, "lon": 13.400000},
+                {"lat": 52.500100, "lon": 13.400050},
+            ],
+            "tags": {
+                "railway": "rail",
+            },
+        }
+    ]
+
+    tracks = tracks_import.normalize_track_elements(elements)
+
+    assert tracks == []