feat: add distance histogram card to observatoire dashboard

Integrate the get_distance_histogram function into the observatoire dashboard
to display buyer-contractor distance distribution on a logarithmic scale.

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

src/pages/observatoire.py

@@ -19,6 +19,7 @@ from dash import (
 
 from src.figures import (
     get_barchart_sources,
+    get_distance_histogram,
     get_duplicate_matrix,
     get_geographic_maps,
     make_card,
@@ -628,6 +629,15 @@ def udpate_dashboard_cards(
         )
     )
 
+    distance_histogram = get_distance_histogram(lff)
+    cards.append(
+        make_card(
+            title="Distance acheteur–titulaire",
+            subtitle="en nombre de marchés, échelle logarithmique",
+            fig=distance_histogram,
+        )
+    )
+
     geographic_maps: list[dbc.Col] = get_geographic_maps(dff)
 
     other_cards = []