Fix db inventory regression: add name-only third fallback

When both table_statistics join and pg_class return all-NULL row_counts,
the notna() guards caused the function to return an empty DataFrame,
triggering the "Could not read table inventory" error. Add a final-resort
query using crdb_internal.tables.estimated_row_count with no notna() guard
so table names always render even when counts are NULL.

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

visualization/debug_page.py

@@ -102,7 +102,8 @@ _LADDER_TB2P_FIELDS = [
 def _query_db_inventory(conn) -> pd.DataFrame:
     """
     List all tables in the public schema with estimated row counts.
-    Uses CockroachDB auto-stats (table_statistics join) with pg_class fallback.
+    Uses CockroachDB auto-stats (table_statistics join) with pg_class fallback,
+    and a final-resort name-only fallback so the table always renders.
     Returns DataFrame with columns [table_name, row_count], sorted by table_name.
     """
     from sqlalchemy import text as _t
@@ -153,6 +154,24 @@ def _query_db_inventory(conn) -> pd.DataFrame:
     except Exception:
         pass
 
+    # Final resort: table names only, row_count will be NULL — better than empty
+    try:
+        df = pd.read_sql(
+            _t("""
+               SELECT name AS table_name,
+                      estimated_row_count AS row_count
+               FROM crdb_internal.tables
+               WHERE schema_name = 'public'
+                 AND table_type = 'table'
+               ORDER BY name
+            """),
+            conn,
+        )
+        if not df.empty:
+            return df
+    except Exception:
+        pass
+
     return pd.DataFrame(columns=["table_name", "row_count"])