OA: Chat message events aggregation

A chat backend wants live counters of how many messages each user sent recently. The service runs for months, so it must forget old events. Build ChatEventAggregator:

• ChatEventAggregator(m: int): the window length in seconds, m >= 1.
• record(user_id: str, chat_id: str, timestamp: int) -> None: one message from user_id in chat_id at timestamp (whole seconds).
• count(user_id: str, timestamp: int) -> int: how many messages user_id sent in the window [timestamp - m + 1, timestamp] (both ends included).
• stored_events() -> int: how many events the aggregator is currently keeping.

Guarantees about the calls:

• The timestamp arguments of all record and count calls, taken together in call order, never decrease (it is a live clock).
• A single user never records twice in the same second (different users can).

Memory rule. After any call with timestamp T, the aggregator must have dropped every event with timestamp <= T - m, for all users, including users who are never mentioned again. stored_events() therefore equals the number of recorded events with timestamp in [T - m + 1, T], where T is the latest timestamp seen (0 before any call). Don't keep empty per-user entries around either.

agg = ChatEventAggregator(10)
agg.record("u1", "c1", 1)
agg.record("u2", "c1", 3)
agg.record("u1", "c2", 5)
agg.count("u1", 5)       # 2
agg.count("u1", 10)      # 2    window [1, 10]
agg.count("u1", 11)      # 1    window [2, 11]: the event at 1 is gone
agg.stored_events()      # 2    (u2@3 and u1@5)
agg.count("u3", 13)      # 0
agg.stored_events()      # 1    (u2@3 expired at T = 13 even though nobody asked about u2)
agg.count("u1", 15)      # 0
agg.stored_events()      # 0

count must be O(1) amortised: don't scan a user's events on every call.