Split into Budget.Core / Budget.Infrastructure / Budget.Api projects

Budget.Core: entities, DTOs, enums, FrequencyCalculator (no EF/ASP.NET deps)
Budget.Infrastructure: AppDbContext, migrations, BudgetAuthorizationService
Budget.Api: controllers, middleware, Program.cs — references both projects

EF and Npgsql packages moved to Infrastructure; Api retains only JwtBearer,
HealthChecks, and EF.Design (needed for dotnet ef CLI). Dockerfile updated
to copy all three project directories before publishing. Migration namespaces
updated from Budget.Api.Data.* to Budget.Infrastructure.Data.* and model
type strings updated to Budget.Core.Models.* in the snapshot.

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

src/Budget.Api/Controllers/BudgetsController.cs

@@ -1,11 +1,13 @@
-using Budget.Api.Data;
-using Budget.Api.DTOs;
-using Budget.Api.Models;
 using Budget.Api.Services;
+using Budget.Core.DTOs;
+using Budget.Core.Models;
+using Budget.Infrastructure.Data;
+using Budget.Infrastructure.Services;
 using Microsoft.AspNetCore.Authorization;
 using Microsoft.AspNetCore.Mvc;
 using Microsoft.AspNetCore.RateLimiting;
 using Microsoft.EntityFrameworkCore;
+using BudgetEntity = Budget.Core.Models.Budget;
 
 namespace Budget.Api.Controllers;
 
@@ -39,7 +41,7 @@ public class BudgetsController(AppDbContext db, BudgetAuthorizationService authz
     public async Task<IActionResult> Create([FromBody] CreateBudgetRequest req)
     {
         if (TryGetUserId(out var userId) is { } err) return err;
-        var budget = new Models.Budget
+        var budget = new BudgetEntity
         {
             Id = Guid.NewGuid(),
             Name = req.Name,