fix(backend): keep chat alive when default-workflow installation fails

WHY THIS MATTERS
buildWorkflowStore() awaited ensureDefaultWorkflows(), which throws on any
RPC error, on every chat message - and the chat routes call it OUTSIDE
their try/catch blocks. Reproduced live: dropping the RPC (equivalent to
deploying code before the migration, or one database hiccup) and sending a
single chat message printed UnhandledPromiseRejection and the Node process
exited. The entire backend went down for every user, and the next message
after a restart killed it again.

WHAT IS AN UNHANDLED REJECTION CRASH?
Express 4 route handlers are plain functions; if you pass an async function
and it throws before you reach your own try block, nothing catches the
rejected promise. Since Node 15, an unhandled promise rejection terminates
the process by default. That turns "one bad query" into "the whole API is
down". (Express 5 forwards async errors to the error middleware; this repo
is on Express 4, so every bare async handler carries this risk.)

HOW THE FIX WORKS
Installing defaults is a convenience, not a prerequisite for chatting, so
the call is now best-effort: failures are logged and the chat proceeds with
whatever workflows already exist. The user-visible defaults still install
on the next successful list/quick-action request, whose asyncRoute wrapper
already converts errors into a 500 JSON response instead of a crash.

Found by fault-injection review of PR #309.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E4PXCdenNH5Mqhm5Sre9Zs