fix: run schema init in its own thread to unblock local db init
When turso is unreachable, schema.init() hangs indefinitely (no
timeouts in zig 0.15 http.Client). Since it was the first thing in
initServices, it blocked local DB init, sync, and all other services.
Spawn schema init in its own thread so local DB can initialize and
serve search queries even when turso is down.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>