feat: add get_distance_histogram figure function

Implement get_distance_histogram that creates a histogram of titulaire distances
with logarithmic scale. Add 3 unit tests covering basic functionality, null handling,
and edge cases. Also add DATA_SCHEMA_PATH to pytest env config.

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

pyproject.toml

@@ -42,6 +42,7 @@ testpaths = [
 ]
 env = [
     "DATA_FILE_PARQUET_PATH=tests/test.parquet",
-    "DEVELOPMENT=true"
+    "DEVELOPMENT=true",
+    "DATA_SCHEMA_PATH=/home/colin/git/decp-processing/dist/schema.json"
 ]
 addopts = "-p no:warnings"