feat: implement remaining API controllers for CRM and tax accounting

Phase 4 Complete: 4 remaining API Controllers
- TaxFilingScheduleController: schedule CRUD + upcoming dues + completion marking
- ConsultingActivityController: activity logging + pending followups + consultant tracking
- ContractController: contract lifecycle + active/expiring tracking + MRR endpoint
- RevenueTrackingController: invoice/payment tracking + pending payments + monthly/total revenue

All controllers follow RESTful patterns with:
- [Authorize] attribute for access control
- Proper error handling with ValidationException catching
- Record-based request/response DTOs
- Consistent HTTP status codes (201, 400, 404, 500)

Build Status: ✅ Success (0 errors, 3 warnings)

TaxBaik.Web/Controllers/ContractController.cs

@@ -0,0 +1,102 @@
+using Microsoft.AspNetCore.Authorization;
+using Microsoft.AspNetCore.Mvc;
+using TaxBaik.Application.Services;
+
+namespace TaxBaik.Web.Controllers;
+
+[ApiController]
+[Route("api/[controller]")]
+[Authorize]
+public class ContractController(ContractService service) : ControllerBase
+{
+    [HttpPost]
+    public async Task<IActionResult> Create([FromBody] CreateContractRequest request)
+    {
+        try
+        {
+            var id = await service.CreateAsync(request.ClientId, request.ContractNumber, request.ServiceType,
+                request.StartDate, request.MonthlyFee, request.TotalAmount);
+            return CreatedAtAction(nameof(GetById), new { id }, new { id });
+        }
+        catch (ValidationException ex)
+        {
+            return BadRequest(new { error = ex.Message });
+        }
+    }
+
+    [HttpGet("{id:int}")]
+    public async Task<IActionResult> GetById(int id)
+    {
+        try
+        {
+            var contract = await service.GetByIdAsync(id);
+            if (contract == null)
+                return NotFound(new { error = "계약을 찾을 수 없습니다." });
+            return Ok(contract);
+        }
+        catch (Exception ex)
+        {
+            return StatusCode(500, new { error = "조회 실패", message = ex.Message });
+        }
+    }
+
+    [HttpGet("client/{clientId:int}")]
+    public async Task<IActionResult> GetByClientId(int clientId)
+    {
+        try
+        {
+            var contracts = await service.GetByClientIdAsync(clientId);
+            return Ok(new { data = contracts });
+        }
+        catch (Exception ex)
+        {
+            return StatusCode(500, new { error = "조회 실패", message = ex.Message });
+        }
+    }
+
+    [HttpGet("active")]
+    public async Task<IActionResult> GetActiveContracts()
+    {
+        try
+        {
+            var contracts = await service.GetActiveContractsAsync();
+            return Ok(new { data = contracts });
+        }
+        catch (Exception ex)
+        {
+            return StatusCode(500, new { error = "조회 실패", message = ex.Message });
+        }
+    }
+
+    [HttpGet("expiring")]
+    public async Task<IActionResult> GetExpiringContracts([FromQuery] int daysAhead = 30)
+    {
+        try
+        {
+            var contracts = await service.GetExpiringContractsAsync(daysAhead);
+            return Ok(new { data = contracts, daysAhead });
+        }
+        catch (Exception ex)
+        {
+            return StatusCode(500, new { error = "조회 실패", message = ex.Message });
+        }
+    }
+
+    [HttpGet("mrr")]
+    public async Task<IActionResult> GetMonthlyRecurringRevenue()
+    {
+        try
+        {
+            var mrr = await service.GetMonthlyRecurringRevenueAsync();
+            return Ok(new { mrr });
+        }
+        catch (Exception ex)
+        {
+            return StatusCode(500, new { error = "조회 실패", message = ex.Message });
+        }
+    }
+
+    public record CreateContractRequest(
+        int ClientId, string ContractNumber, string ServiceType, DateTime StartDate,
+        decimal? MonthlyFee = null, decimal? TotalAmount = null);
+}