SessionStore._entries grew unbounded. Every unique
(platform, chat_id, thread_id, user_id) tuple ever seen was kept in
RAM and rewritten to sessions.json on every message. A Discord bot
in 100 servers x 100 channels x ~100 rotating users accumulates on
the order of 10^5 entries after a few months; each sessions.json
write becomes an O(n) fsync. Nothing trimmed this — there was no
TTL, no cap, no eviction path.
Changes
-------
* SessionStore.prune_old_entries(max_age_days) — drops entries whose
updated_at is older than the cutoff. Preserves:
- suspended entries (user paused them via /stop for later resume)
- entries with an active background process attached
Pruning is functionally identical to a natural reset-policy expiry:
SQLite transcript stays, session_key -> session_id mapping dropped,
returning user gets a fresh session.
* GatewayConfig.session_store_max_age_days (default 90; 0 disables).
Serialized in to_dict/from_dict, coerced from bad types / negatives
to safe defaults. No migration needed — missing field -> 90 days.
* _session_expiry_watcher calls prune_old_entries once per hour
(first tick is immediate). Uses the existing watcher loop so no
new background task is created.
Why not more aggressive
-----------------------
90 days is long enough that legitimate long-idle users (seasonal,
vacation, etc.) aren't surprised — pruning just means they get a
fresh session on return, same outcome they'd get from any other
reset-policy trigger. Admins can lower it via config; 0 disables.
Tests
-----
tests/gateway/test_session_store_prune.py — 17 cases covering:
* entry age based on updated_at, not created_at
* max_age_days=0 disables; negative coerces to 0
* suspended + active-process entries are skipped
* _save fires iff something was removed
* disk JSON reflects post-prune state
* thread safety against concurrent readers
* config field roundtrips + graceful fallback on bad values
* watcher gate logic (first tick prunes, subsequent within 1h don't)
119 broader session/gateway tests remain green.
