Server no longer holds one fixed Garmin/Sync pair -- garminFor/syncFor
build and cache one instance per user, keyed off their own profile's
Garmin credentials and a per-user token-store subdirectory. The background
incremental sync loop now iterates every provisioned user each tick
instead of syncing one global account.
resolveUser attaches the session's provisioned geniusrun user (if any) to
request context without blocking; requireProvisionedUser (wired fully in
Task 13) 403s routes that need one. GET /api/session/me now reports
has_profile/display_name so the frontend can show the setup screen.
