feat(fleet): add service type enum and infra scope

Replace the free-form service_type string with a ServiceType enum (ccms,
ccmc, ccb, cces, ccsa, ccnc, ccem). The short code is the canonical value
stored in the DB, the config-tree directory name and a NATS discovery
subject token; Description() gives the human-readable name.

Add a second scope alongside per-node agents: ScopeInfra covers
cluster-independent monitoring infrastructure services, which are
identified by (hostname, service_type) and carry an empty cluster.
InfraRegistry is the ScopeInfra sibling of Registry and shares the same
table, state machine and config-revision handshake. It deliberately
exposes no StartSweep: MarkStale ages rows by last_heartbeat regardless
of scope, so a single sweep goroutine per process covers both scopes.

On the repository side ServiceDB gains a Scope column and the two
queries the discovery publisher and infra enumeration need, ListActive()
and ListByScope(). ResetConnection() now also resets the fleet
repository singleton so tests get a fresh handle.

Migration 13 is edited in place rather than superseded by a new
migration: the service table is unreleased, so no existing deployment
has it yet.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

internal/repository/migration.go

@@ -21,7 +21,8 @@ import (
 // is added to internal/repository/migrations/sqlite3/.
 //
 // Version history:
-//   - Version 13: Service table (fleet service registration/heartbeat)
+//   - Version 13: Service table (fleet service registration/heartbeat), incl.
+//     scope column (cluster vs cluster-independent infra services)
 //   - Version 12: Sessions table (server-side sessions via alexedwards/scs)
 //   - Version 11: Optimize job table indexes (reduce from ~78 to 48, add covering/partial indexes)
 //   - Version 10: Node table