feat: migrate TaxFilingScheduleController to FastEndpoints (Phase 6)

IMPLEMENTATION:
- Create 7 FastEndpoints Endpoints:
  - CreateEndpoint, GetByIdEndpoint, GetAllEndpoint
  - GetByClientIdEndpoint, GetUpcomingEndpoint
  - MarkCompletedEndpoint, GetPendingCountEndpoint

Total: 38 endpoints migrated (out of 73)
Remaining: 12 Controllers (35 endpoints)

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

src/TaxBaik.Web/Controllers/TaxFilingScheduleController.cs.bak

@@ -0,0 +1,116 @@
+using Microsoft.AspNetCore.Authorization;
+using Microsoft.AspNetCore.Mvc;
+using TaxBaik.Application.Services;
+
+namespace TaxBaik.Web.Controllers;
+
+[ApiController]
+[Route("api/[controller]")]
+[Authorize]
+public class TaxFilingScheduleController(TaxFilingScheduleService service) : ControllerBase
+{
+    [HttpPost]
+    public async Task<IActionResult> Create([FromBody] CreateTaxFilingScheduleRequest request)
+    {
+        try
+        {
+            var id = await service.CreateAsync(request.ClientId, request.FilingType, request.DueDate,
+                request.FilingYear, request.AssignedTo);
+            return CreatedAtAction(nameof(GetById), new { id }, new { id });
+        }
+        catch (ValidationException ex)
+        {
+            return BadRequest(new { error = ex.Message });
+        }
+    }
+
+    [HttpGet]
+    public async Task<IActionResult> GetAll()
+    {
+        try
+        {
+            var schedules = await service.GetAllAsync();
+            return Ok(schedules);
+        }
+        catch (Exception ex)
+        {
+            return StatusCode(500, new { error = "조회 실패", message = ex.Message });
+        }
+    }
+
+    [HttpGet("{id:int}")]
+    public async Task<IActionResult> GetById(int id)
+    {
+        try
+        {
+            var schedule = await service.GetByIdAsync(id);
+            if (schedule == null)
+                return NotFound(new { error = "신고 일정을 찾을 수 없습니다." });
+            return Ok(schedule);
+        }
+        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 schedules = await service.GetByClientIdAsync(clientId);
+            return Ok(new { data = schedules });
+        }
+        catch (Exception ex)
+        {
+            return StatusCode(500, new { error = "조회 실패", message = ex.Message });
+        }
+    }
+
+    [HttpGet("upcoming")]
+    public async Task<IActionResult> GetUpcomingDues([FromQuery] int daysAhead = 30)
+    {
+        try
+        {
+            var schedules = await service.GetUpcomingDuesAsync(daysAhead);
+            return Ok(new { data = schedules, daysAhead });
+        }
+        catch (Exception ex)
+        {
+            return StatusCode(500, new { error = "조회 실패", message = ex.Message });
+        }
+    }
+
+    [HttpGet("pending-count")]
+    public async Task<IActionResult> GetPendingCount()
+    {
+        try
+        {
+            var count = await service.GetPendingCountAsync();
+            return Ok(new { count });
+        }
+        catch (Exception ex)
+        {
+            return StatusCode(500, new { error = "조회 실패", message = ex.Message });
+        }
+    }
+
+    [HttpPut("{id:int}/complete")]
+    public async Task<IActionResult> MarkCompleted(int id)
+    {
+        try
+        {
+            await service.MarkCompletedAsync(id);
+            return Ok(new { message = "신고 일정이 완료되었습니다." });
+        }
+        catch (Exception ex)
+        {
+            return StatusCode(500, new { error = "수정 실패", message = ex.Message });
+        }
+    }
+
+    public record CreateTaxFilingScheduleRequest(
+        int ClientId, string FilingType, DateTime DueDate, int FilingYear,
+        int? AssignedTo = null);
+}