Add audit log infrastructure and GET /v1/audit-logs endpoint

Introduces an append-only audit trail for all user and system actions:
sandbox lifecycle (create/pause/resume/destroy/auto-pause), snapshots,
team rename, API key create/revoke, member add/remove/leave/role_update,
and BYOC host add/delete/marked_down/marked_up.

- New audit_logs table (migration) with team_id, actor, resource,
  action, scope (team|admin), status (success|info|warning|error),
  metadata, and created_at
- AuditLogger (internal/audit) with named fire-and-forget methods per
  event; system actor used for background events (HostMonitor, TTL reaper)
- GET /v1/audit-logs: JWT-only, cursor pagination (max 200), multi-value
  filters for resource_type and action (comma-sep or repeated params);
  members see team-scoped events only, admins/owners see all
- AuthContext extended with APIKeyID + APIKeyName so API key requests
  record meaningful actor identity
- HostMonitor wired with AuditLogger for auto-pause and host marked_down

internal/id/id.go

@@ -73,6 +73,11 @@ func NewRefreshTokenID() string {
 	return "hrt-" + hex8()
 }
 
+// NewAuditLogID generates a new audit log ID in the format "log-" + 8 hex chars.
+func NewAuditLogID() string {
+	return "log-" + hex8()
+}
+
 // NewRefreshToken generates a 64-char hex token (32 bytes of entropy) for use as a host refresh token.
 func NewRefreshToken() string {
 	b := make([]byte, 32)