CUB-54: implement AgentState entity, repository, and DI registration

- Add AgentState entity mapping to 'agents' table (snake_case columns)
- Add IAgentStateRepository interface (GetAllAsync, GetBySessionKeyAsync, UpdateStatusAsync)
- Add AgentStateRepository with EF Core implementation
- Add ControlCenterDbContext with ApplyConfigurationsFromAssembly
- Add AgentStateConfiguration with snake_case column mappings and indexes
- Register DbContext (Npgsql) and repository in Program.cs DI
- Add ConnectionStrings to appsettings.json
- Add EF Core 9.0.7 and Npgsql.EntityFrameworkCore.PostgreSQL 9.0.4 packages

backend/ControlCenter/appsettings.json

@@ -8,6 +8,10 @@
   },
   "AllowedHosts": "*",
 
+  "ConnectionStrings": {
+    "ControlCenterDb": "Host=localhost;Port=5432;Database=control_center;Username=control_center;Password=changeme"
+  },
+
   "Gateway": {
     "WebSocketUrl": "ws://localhost:3271/ws",
     "AuthToken": ""