chore: rename 'last_used_at' column (#26749)

Renames the `last_used_at` column  to `last_heartbeat_at` in `ai_gateway_keys` table.  
`ai_gateway_keys` table has not been released yet.  
All references updated.

coderd/database/queries/ai_gateway_keys.sql

@@ -4,13 +4,13 @@ VALUES ($1, @name, $2, $3, NOW())
 RETURNING id, name, secret_prefix, created_at;
 
 -- name: ListAIGatewayKeys :many
-SELECT id, name, secret_prefix, created_at, last_used_at
+SELECT id, name, secret_prefix, created_at, last_heartbeat_at
 FROM ai_gateway_keys
 ORDER BY created_at ASC;
 
 -- name: DeleteAIGatewayKey :one
 DELETE FROM ai_gateway_keys WHERE id = $1
-RETURNING id, name, secret_prefix, created_at, last_used_at;
+RETURNING id, name, secret_prefix, created_at, last_heartbeat_at;
 
 -- name: GetAIGatewayKeyByHashedSecret :one
 -- Authenticates a standalone AI Gateway replica by its hashed key secret,
@@ -20,10 +20,10 @@ SELECT *
 FROM ai_gateway_keys
 WHERE hashed_secret = $1;
 
--- name: UpdateAIGatewayKeyLastUsedAt :execrows
--- Records liveness for an active Gateway DRPC session. The database sets the
+-- name: UpdateAIGatewayKeyLastHeartbeatAt :execrows
+-- Records heartbeat liveness for an active Gateway DRPC session. The database sets the
 -- timestamp so it stays consistent regardless of clock drift between API
 -- replicas.
 UPDATE ai_gateway_keys
-SET last_used_at = NOW()
+SET last_heartbeat_at = NOW()
 WHERE id = $1;