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/app/core/osm_config.py

@@ -76,17 +76,42 @@ STATION_TAG_FILTERS: Mapping[str, Tuple[str, ...]] = {
 
 
 # Tags that describe rail infrastructure usable for train routing.
+TRACK_ALLOWED_RAILWAY_TYPES: Tuple[str, ...] = (
+    "rail",
+    "light_rail",
+    "subway",
+    "tram",
+    "narrow_gauge",
+    "disused",
+    "construction",
+)
+
+
 TRACK_TAG_FILTERS: Mapping[str, Tuple[str, ...]] = {
-    "railway": (
-        "rail",
-        "light_rail",
-        "subway",
-        "tram",
-        "narrow_gauge",
-    ),
+    "railway": TRACK_ALLOWED_RAILWAY_TYPES,
 }
 
 
+# Track ingestion policy
+TRACK_EXCLUDED_SERVICE_TAGS: Tuple[str, ...] = (
+    "yard",
+    "siding",
+    "spur",
+    "crossover",
+    "industrial",
+    "military",
+)
+
+TRACK_EXCLUDED_USAGE_TAGS: Tuple[str, ...] = (
+    "military",
+    "tourism",
+)
+
+TRACK_MIN_LENGTH_METERS: float = 75.0
+
+TRACK_STATION_SNAP_RADIUS_METERS: float = 350.0
+
+
 def compile_overpass_filters(filters: Mapping[str, Iterable[str]]) -> str:
     """Build an Overpass boolean expression that matches the provided filters."""
 
@@ -101,6 +126,11 @@ __all__ = [
     "BoundingBox",
     "DEFAULT_REGIONS",
     "STATION_TAG_FILTERS",
+    "TRACK_ALLOWED_RAILWAY_TYPES",
     "TRACK_TAG_FILTERS",
+    "TRACK_EXCLUDED_SERVICE_TAGS",
+    "TRACK_EXCLUDED_USAGE_TAGS",
+    "TRACK_MIN_LENGTH_METERS",
+    "TRACK_STATION_SNAP_RADIUS_METERS",
     "compile_overpass_filters",
 ]