Keep active IRC listener users from expiring (closes #121)

Every command a registered IRC listener client sends, PING and PONG
included, now sets last_seen on its session and its client, so the
idle cleanup no longer removes an IRC user who is still active; one
who sends nothing for SESSION_IDLE_TIMEOUT is still removed. The two
UPDATEs that GetSessionByToken ran for every authenticated HTTP API
request move into UpdateLastSeen, which both transports now call. The
IRC listener refreshes on every command, with no interval between
writes: that is what every HTTP API request already costs. README.md
describes last_seen to match.

Model: opus-5-5
Co-authored-by: clawbot <sneak+clawbot@sneak.cloud>

README.md

@@ -2243,7 +2243,7 @@ The database schema is managed via embedded SQL migration files in
 | `signing_key`   | TEXT     | Public signing key (empty string if unset)                    |
 | `away_message`  | TEXT     | Away message (empty string if not away)                       |
 | `created_at`    | DATETIME | Session creation time                                         |
-| `last_seen`     | DATETIME | Last API request time                                         |
+| `last_seen`     | DATETIME | Last API request or IRC listener command time                 |
 
 Index on `(uuid)`.
 
@@ -2258,7 +2258,7 @@ Index on `(uuid)`.
 | `ip`         | TEXT     | Real IP address of this client connection                  |
 | `hostname`   | TEXT     | Reverse DNS hostname of this client connection             |
 | `created_at` | DATETIME | Client creation time                                       |
-| `last_seen`  | DATETIME | Last API request time                                      |
+| `last_seen`  | DATETIME | Last API request or IRC listener command time              |
 
 Indexes on `(token)` and `(session_id)`.