Phase 3C: Deployment pipeline & Store-and-Forward engine

Deployment Manager (WP-1–8, WP-16):
- DeploymentService: full pipeline (flatten→validate→send→track→audit)
- OperationLockManager: per-instance concurrency control
- StateTransitionValidator: Enabled/Disabled/NotDeployed transition matrix
- ArtifactDeploymentService: broadcast to all sites with per-site results
- Deployment identity (GUID + revision hash), idempotency, staleness detection
- Instance lifecycle commands (disable/enable/delete) with deduplication

Store-and-Forward (WP-9–15):
- StoreAndForwardStorage: SQLite persistence, 3 categories, no max buffer
- StoreAndForwardService: fixed-interval retry, transient-only buffering, parking
- ReplicationService: async best-effort to standby (fire-and-forget)
- Parked message management (query/retry/discard from central)
- Messages survive instance deletion, S&F drains on disable

620 tests pass (+79 new), zero warnings.

src/ScadaLink.ConfigurationDatabase/ScadaLinkDbContext.cs

@@ -40,6 +40,7 @@ public class ScadaLinkDbContext : DbContext, IDataProtectionKeyContext
     // Deployment
     public DbSet<DeploymentRecord> DeploymentRecords => Set<DeploymentRecord>();
     public DbSet<SystemArtifactDeploymentRecord> SystemArtifactDeploymentRecords => Set<SystemArtifactDeploymentRecord>();
+    public DbSet<DeployedConfigSnapshot> DeployedConfigSnapshots => Set<DeployedConfigSnapshot>();
 
     // External Systems
     public DbSet<ExternalSystemDefinition> ExternalSystemDefinitions => Set<ExternalSystemDefinition>();