Add DID-less startup tests and fix eager schema initialization
9 tests verify: server starts without DID, dashboard/health work,
replication manager exists, can add/sync DIDs, node_identity table
persists identity across restarts, env DID overrides stored identity.
Fixes: SyncStorage/ChallengeStorage schemas now init eagerly in
ReplicationManager constructor (not deferred to async init()), and
backfillIpfs skips when no RepoManager exists.