Sync endpoint returns 500 with unhelpful error when gateway not running

[isaacrowntree]

Problem

POST /api/admin/storage/sync returns HTTP 500 with "Sync aborted: no config file found" when the gateway process hasn't started yet (or when config files are missing for other reasons). The error gives no context about why the config is missing.

Related to #207 — the cron sync handler already guards against this ([cron] Gateway not running yet, skipping sync), but the manual sync endpoint doesn't.

Expected behaviour

• Return 409 (not 500) when gateway isn't running — it's a precondition, not a server error
• When config files are missing but the gateway IS running, provide diagnostic info: what's in the config directories, what's in the R2 backup, whether R2 is responsive
• Return 400 for other client-side errors like "R2 not configured"

Related

• R2 backup never runs: requires active gateway process, creating chicken-and-egg problem #207 — R2 backup never runs when gateway not active
• Handle 'already in use' error when R2 bucket is already mounted #209 — alternative approach: start gateway before sync
• Admin UI: show gateway status and improve controls #213 — show gateway status in admin UI

[isaacrowntree]

Addressed by #211 (sync diagnostics) and #215 (root cause fix).

• fix: add gateway check and diagnostics to sync endpoint #211 adds diagnostics when sync fails and a gateway-running pre-check (returns 409 instead of 500)
• fix: use stdout-based config dir detection instead of exitCode #215 fixes the underlying exitCode null bug that causes sync to always fail