Set page title server-side for link preview compatibility

When a specific relation URL is loaded, fetch_relation_name() makes a
lightweight GET /relation/{id}.json call to get the name tag and pass
it to the template. The <title> and og:title are then set server-side,
so forum link previews and crawlers see the route name in the HTML
without executing JavaScript.

Errors are swallowed silently so a slow or failed API response just
falls back to the default title.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/osm_geojson/pt/core.py

@@ -119,6 +119,27 @@ def nearest_coord_index(lon: float, lat: float, route_coords: list[Coord]) -> in
     return best_i
 
 
+def fetch_relation_name(relation_id: int) -> str | None:
+    """Return the name tag of a relation, or None if unavailable.
+
+    Uses a short timeout and swallows all errors — intended for best-effort
+    use in page titles where a fallback is acceptable.
+    """
+    url = f"{OSM_API}/relation/{relation_id}.json"
+    try:
+        resp = requests.get(url, headers={"User-Agent": "osm-pt-geojson/1.0"}, timeout=10)
+    except requests.RequestException:
+        return None
+    if resp.status_code != 200:
+        return None
+    data: dict[str, Any] = resp.json()
+    for elem in data.get("elements", []):
+        if elem["type"] == "relation" and elem["id"] == relation_id:
+            name: str | None = elem.get("tags", {}).get("name")
+            return name
+    return None
+
+
 def fetch_sibling_routes(relation_id: int) -> list[dict[str, Any]]:
     """Return sibling route relations from the same route_master, excluding self.