feat: add data_date to project summary response

Include the schedule data date (last_recalc_date from XER) in the
get_project_summary tool response. This shows when the schedule
was last calculated in P6.

Changes:
- Add last_recalc_date column to projects table schema
- Parse last_recalc_date in PROJECT table handler
- Include last_recalc_date in db loader
- Return data_date field in get_project_summary query
- Update contract test to verify data_date presence

src/xer_mcp/db/queries.py

@@ -290,7 +290,7 @@ def get_project_summary(project_id: str) -> dict | None:
     with db.cursor() as cur:
         cur.execute(
             """
-            SELECT proj_id, proj_short_name, plan_start_date, plan_end_date
+            SELECT proj_id, proj_short_name, plan_start_date, plan_end_date, last_recalc_date
             FROM projects
             WHERE proj_id = ?
             """,
@@ -319,6 +319,7 @@ def get_project_summary(project_id: str) -> dict | None:
     return {
         "project_id": project_row[0],
         "project_name": project_row[1],
+        "data_date": project_row[4],
         "plan_start_date": project_row[2],
         "plan_end_date": project_row[3],
         "activity_count": activity_count,