feat: integrate platform backend and application interfaces

server/migrations/0017_v1_webdav_job_leases.sql

@@ -0,0 +1,5 @@
+ALTER TABLE webdav_sync_jobs
+  ADD COLUMN IF NOT EXISTS lease_owner TEXT;
+
+CREATE INDEX IF NOT EXISTS webdav_sync_jobs_claim_idx
+  ON webdav_sync_jobs(status, next_attempt_at, lease_expires_at, created_at);