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/scripts/tracks_import.py

@@ -8,11 +8,15 @@ import math
 import sys
 from dataclasses import asdict
 from pathlib import Path
-from typing import Any, Iterable
+from typing import Any, Iterable, Mapping
 from urllib.parse import quote_plus
 
 from backend.app.core.osm_config import (
     DEFAULT_REGIONS,
+    TRACK_ALLOWED_RAILWAY_TYPES,
+    TRACK_EXCLUDED_SERVICE_TAGS,
+    TRACK_EXCLUDED_USAGE_TAGS,
+    TRACK_MIN_LENGTH_METERS,
     TRACK_TAG_FILTERS,
     compile_overpass_filters,
 )
@@ -104,13 +108,15 @@ def normalize_track_elements(elements: Iterable[dict[str, Any]]) -> list[dict[st
             continue
 
         tags: dict[str, Any] = element.get("tags", {})
+        length_meters = _polyline_length(coordinates)
+        if not _should_include_track(tags, length_meters):
+            continue
+
         name = tags.get("name")
         maxspeed = _parse_maxspeed(tags.get("maxspeed"))
         status = _derive_status(tags.get("railway"))
         is_bidirectional = not _is_oneway(tags.get("oneway"))
 
-        length_meters = _polyline_length(coordinates)
-
         tracks.append(
             {
                 "osmId": str(element.get("id")),
@@ -156,6 +162,25 @@ def _derive_status(value: Any) -> str:
     return "operational"
 
 
+def _should_include_track(tags: Mapping[str, Any], length_meters: float) -> bool:
+    railway = str(tags.get("railway", "")).lower()
+    if railway not in TRACK_ALLOWED_RAILWAY_TYPES:
+        return False
+
+    if length_meters < TRACK_MIN_LENGTH_METERS:
+        return False
+
+    service = str(tags.get("service", "")).lower()
+    if service and service in TRACK_EXCLUDED_SERVICE_TAGS:
+        return False
+
+    usage = str(tags.get("usage", "")).lower()
+    if usage and usage in TRACK_EXCLUDED_USAGE_TAGS:
+        return False
+
+    return True
+
+
 def _is_oneway(value: Any) -> bool:
     if value is None:
         return False