Phase 3: Budget and sharing API

- Add BudgetsController: list (owner + shared), create, get, rename, delete
- Add BudgetAuthorizationService: Owner / Edit / View / None access levels
- Add SharesController: list, add (resolves KnownUser immediately), update permission, revoke
- Register BudgetAuthorizationService as scoped service
- Add BudgetDto, ShareDto, and associated request DTOs

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/Budget.Api/Program.cs

@@ -29,6 +29,7 @@ builder.Services.AddAuthentication(JwtBearerDefaults.AuthenticationScheme)
     });
 
 builder.Services.AddAuthorization();
+builder.Services.AddScoped<Budget.Api.Services.BudgetAuthorizationService>();
 builder.Services.AddControllers();
 
 var app = builder.Build();