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/schema.py

@@ -7,6 +7,7 @@ CREATE TABLE IF NOT EXISTS projects (
     proj_short_name TEXT NOT NULL,
     plan_start_date TEXT,
     plan_end_date TEXT,
+    last_recalc_date TEXT,
     loaded_at TEXT NOT NULL DEFAULT (datetime('now'))
 );