Transform clickstream with pandas sessionization

Given a pandas DataFrame events with columns [user_id:int, ts:str ISO8601 or NaT, url:str, server_log_ts:datetime], build 30-minute inactivity sessions per user: 1) Use server_log_ts to impute ts when ts is missing; 2) Robustly sort events per user with potentially out-of-order rows; 3) Define session_id when the gap > 30 minutes; 4) Compute, for each user, session_count, median_session_duration, and the 95th percentile of pages per session; 5) Ensure the solution works in streaming-sized chunks (cannot load all users into memory). Provide vectorized code sketches and explain correctness on edge cases (exactly-30-minute gaps, duplicated events, DST shifts).