feat(store): drift state, move candidates and service merge (#10)

Adds services.awaiting_inventory, Service.Drifted/RunsRef/RunningRef, AcceptRunning, MoveCandidates, MergeSources and MergeService, and maps a job for a deleted service to ErrNotFound.

internal/master/store/migrations/0005_drift_merge.sql

@@ -0,0 +1,7 @@
+-- true from a successful job until the next inventory write for the row: the
+-- agent's upsert arrives a debounce interval after job.result, and until then
+-- desired_ref is newer than actual_ref without the service having drifted.
+ALTER TABLE services ADD COLUMN awaiting_inventory boolean NOT NULL DEFAULT false;
+
+-- move suggestions look up Compose rows by project + service across agents
+CREATE INDEX services_by_compose_name ON services (compose_project, compose_service) WHERE kind = 'compose';