feat(pseo): write _serving_meta.json after atomic serving DB swap

Records exported_at_utc timestamp and per-table row counts immediately
after export_serving.py completes. The pSEO Engine dashboard reads this
file to show data freshness without querying file mtimes.

Also moves the inline `import re` to the top-level imports.

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

src/padelnomics/export_serving.py

@@ -24,8 +24,12 @@ Usage:
         uv run python -m padelnomics.export_serving
 """
 
+import json
 import logging
 import os
+import re
+from datetime import UTC, datetime
+from pathlib import Path
 
 import duckdb
 
@@ -44,6 +48,8 @@ def export_serving() -> None:
     # (rename across filesystems is not atomic on Linux).
     tmp_path = os.path.join(os.path.dirname(os.path.abspath(serving_path)), "_export.duckdb")
 
+    table_counts: dict[str, int] = {}
+
     src = duckdb.connect(pipeline_path, read_only=True)
     try:
         # SQLMesh creates serving views that reference "local".sqlmesh__serving.*
@@ -60,7 +66,6 @@ def export_serving() -> None:
         for view_name, view_sql in view_rows:
             # Pattern: ... FROM "local".sqlmesh__serving.serving__name__hash;
             # Strip the "local". prefix to get schema.table
-            import re
             match = re.search(r'FROM\s+"local"\.(sqlmesh__serving\.\S+)', view_sql)
             assert match, f"Cannot parse view definition for {view_name}: {view_sql[:200]}"
             physical_tables.append((view_name, match.group(1)))
@@ -81,6 +86,7 @@ def export_serving() -> None:
                 dst.execute(f"CREATE OR REPLACE TABLE serving.{logical_name} AS SELECT * FROM _src")
                 dst.unregister("_src")
                 row_count = dst.sql(f"SELECT count(*) FROM serving.{logical_name}").fetchone()[0]
+                table_counts[logical_name] = row_count
                 logger.info(f"  serving.{logical_name}: {row_count:,} rows")
         finally:
             dst.close()
@@ -91,6 +97,16 @@ def export_serving() -> None:
     os.rename(tmp_path, serving_path)
     logger.info(f"Serving DB atomically updated: {serving_path}")
 
+    # Write freshness metadata so the pSEO dashboard can show data age without
+    # querying file mtimes (which are unreliable after rclone syncs).
+    meta_path = Path(serving_path).parent / "_serving_meta.json"
+    meta = {
+        "exported_at_utc": datetime.now(tz=UTC).isoformat(),
+        "tables": {name: {"row_count": count} for name, count in table_counts.items()},
+    }
+    meta_path.write_text(json.dumps(meta))
+    logger.info("Wrote serving metadata: %s", meta_path)
+
 
 if __name__ == "__main__":
     logging.basicConfig(level=logging.INFO, format="%(asctime)s %(name)s %(levelname)s %(message)s")