feat: add warning expiration and exclude moderated messages from context

Warning flag now auto-expires after a configurable duration
(warning_expiration_minutes, default 30m). After expiry, the user must
be re-warned before a mute can be issued.

Messages that triggered moderation actions (warnings/mutes) are now
excluded from the LLM context window in both buffered analysis and
mention scans, preventing already-actioned content from influencing
future scoring. Uses in-memory tracking plus bot reaction fallback
for post-restart coverage.

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

cogs/sentiment/state.py

@@ -22,6 +22,7 @@ def save_user_state(bot, dirty_users: set[int], user_id: int) -> None:
         warned=user_data.warned_since_reset,
         last_offense_at=user_data.last_offense_time or None,
         aliases=_aliases_csv(user_data),
+        warning_expires_at=user_data.warning_expires_at or None,
     ))
     dirty_users.discard(user_id)
 
@@ -44,5 +45,6 @@ async def flush_dirty_states(bot, dirty_users: set[int]) -> None:
             warned=user_data.warned_since_reset,
             last_offense_at=user_data.last_offense_time or None,
             aliases=_aliases_csv(user_data),
+            warning_expires_at=user_data.warning_expires_at or None,
         )
     logger.info("Flushed %d dirty user states to DB.", len(dirty))