Add per-table TTL refresh for tables without a change column

src/sqlmem/cache.py

@@ -143,6 +143,16 @@ class CacheManager:
         ).fetchone()
         return bool(row and row[0])
 
+    def seconds_since_refresh(self, table: str) -> float | None:
+        """Age of a cached table in seconds, or None if it is not cached."""
+        row = self._mem_conn.execute(
+            "SELECT last_refresh_at FROM _sqlmem_tables WHERE table_name = ?", (table,)
+        ).fetchone()
+        if not row or not row[0]:
+            return None
+        last = datetime.fromisoformat(row[0])
+        return (datetime.now(timezone.utc) - last).total_seconds()
+
     def discover_columns(self, table: str, source_conn: sqlite3.Connection) -> list[str]:
         """Return all column names of *table* from the source DB without fetching rows."""
         logger.debug(f"Discovering columns of {table!r} from source DB")