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/ConsultingActivityController.cs

@@ -0,0 +1,106 @@
+using Microsoft.AspNetCore.Authorization;
+using Microsoft.AspNetCore.Mvc;
+using TaxBaik.Application.Services;
+
+namespace TaxBaik.Web.Controllers;
+
+[ApiController]
+[Route("api/[controller]")]
+[Authorize]
+public class ConsultingActivityController(ConsultingActivityService service) : ControllerBase
+{
+    [HttpPost]
+    public async Task<IActionResult> Create([FromBody] CreateConsultingActivityRequest request)
+    {
+        try
+        {
+            var id = await service.CreateAsync(request.ClientId, request.ActivityType, request.ActivityDate,
+                request.Description, request.ConsultantId, request.NextFollowupDate);
+            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 activity = await service.GetByClientIdAsync(id);
+            if (activity == null)
+                return NotFound(new { error = "상담 활동을 찾을 수 없습니다." });
+            return Ok(activity);
+        }
+        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 activities = await service.GetByClientIdAsync(clientId);
+            return Ok(new { data = activities });
+        }
+        catch (Exception ex)
+        {
+            return StatusCode(500, new { error = "조회 실패", message = ex.Message });
+        }
+    }
+
+    [HttpGet("pending-followups")]
+    public async Task<IActionResult> GetPendingFollowups()
+    {
+        try
+        {
+            var activities = await service.GetPendingFollowupsAsync();
+            return Ok(new { data = activities });
+        }
+        catch (Exception ex)
+        {
+            return StatusCode(500, new { error = "조회 실패", message = ex.Message });
+        }
+    }
+
+    [HttpGet("consultant/{consultantId:int}")]
+    public async Task<IActionResult> GetByConsultant(int consultantId, [FromQuery] int daysBack = 30)
+    {
+        try
+        {
+            var fromDate = DateTime.Today.AddDays(-daysBack);
+            var activities = await service.GetConsultantActivityAsync(consultantId, fromDate);
+            return Ok(new { data = activities, daysBack });
+        }
+        catch (Exception ex)
+        {
+            return StatusCode(500, new { error = "조회 실패", message = ex.Message });
+        }
+    }
+
+    [HttpPut("{id:int}")]
+    public async Task<IActionResult> Update(int id, [FromBody] UpdateConsultingActivityRequest request)
+    {
+        try
+        {
+            await service.UpdateAsync(id, request.Outcome, request.NextFollowupDate);
+            return Ok(new { message = "상담 활동이 수정되었습니다." });
+        }
+        catch (Exception ex)
+        {
+            return StatusCode(500, new { error = "수정 실패", message = ex.Message });
+        }
+    }
+
+    public record CreateConsultingActivityRequest(
+        int ClientId, string ActivityType, DateTime ActivityDate, string Description,
+        int? ConsultantId = null, DateTime? NextFollowupDate = null);
+
+    public record UpdateConsultingActivityRequest(
+        string? Outcome = null, DateTime? NextFollowupDate = null);
+}