feat(mcp): подготовить сервер и доступ к приёмке

backend/drizzle/0027_mcp_readiness.sql

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+CREATE INDEX IF NOT EXISTS mcp_audit_handshake_cooldown
+  ON mcp_audit(connection_id, created_at DESC)
+  WHERE action = 'connection.handshake.start';
+CREATE INDEX IF NOT EXISTS mcp_credentials_readiness_expired
+  ON mcp_credentials(connection_id, expires_at)
+  WHERE client_id = 'mcp-readiness-probe';
+CREATE INDEX IF NOT EXISTS mcp_grants_credential
+  ON mcp_grants(connection_id, (payload->>'credentialId'));