fix(gateway): prune stale SessionStore entries to bound memory + disk (#11789)

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.

gateway/config.py

@@ -258,6 +258,13 @@ class GatewayConfig:
     # Streaming configuration
     streaming: StreamingConfig = field(default_factory=StreamingConfig)
 
+    # Session store pruning: drop SessionEntry records older than this many
+    # days from the in-memory dict and sessions.json.  Keeps the store from
+    # growing unbounded in gateways serving many chats/threads/users over
+    # months.  Pruning is invisible to users — if they resume, they get a
+    # fresh session exactly as if the reset policy had fired.  0 = disabled.
+    session_store_max_age_days: int = 90
+
     def get_connected_platforms(self) -> List[Platform]:
         """Return list of platforms that are enabled and configured."""
         connected = []
@@ -365,6 +372,7 @@ class GatewayConfig:
             "thread_sessions_per_user": self.thread_sessions_per_user,
             "unauthorized_dm_behavior": self.unauthorized_dm_behavior,
             "streaming": self.streaming.to_dict(),
+            "session_store_max_age_days": self.session_store_max_age_days,
         }
     
     @classmethod
@@ -412,6 +420,13 @@ class GatewayConfig:
             "pair",
         )
 
+        try:
+            session_store_max_age_days = int(data.get("session_store_max_age_days", 90))
+            if session_store_max_age_days < 0:
+                session_store_max_age_days = 0
+        except (TypeError, ValueError):
+            session_store_max_age_days = 90
+
         return cls(
             platforms=platforms,
             default_reset_policy=default_policy,
@@ -426,6 +441,7 @@ class GatewayConfig:
             thread_sessions_per_user=_coerce_bool(thread_sessions_per_user, False),
             unauthorized_dm_behavior=unauthorized_dm_behavior,
             streaming=StreamingConfig.from_dict(data.get("streaming", {})),
+            session_store_max_age_days=session_store_max_age_days,
         )
 
     def get_unauthorized_dm_behavior(self, platform: Optional[Platform] = None) -> str: