refactor: Phase 7-3 - Clients + TaxFilings API-First (WIP)

**Clients Migration Complete:**
- ClientController: GET /api/client (paged), POST/PUT/DELETE
- ClientBrowserClient: IClientBrowserClient interface + implementation
- ClientList.razor: Service → API client
- ClientEdit.razor: Service → API client (Create/Update)

**TaxFilings API Framework Ready:**
- TaxFilingController: GET upcoming, GET by client, POST/PUT/DELETE
- TaxFilingBrowserClient: ITaxFilingBrowserClient interface + impl
- Registered in Program.cs with TokenRefreshHandler

**SOLID Applied:**
✓ Separation of concerns (Controller → Service → Repository)
✓ Dependency inversion (Blazor → Browser clients, not services)
✓ Interface segregation (Specialized clients per domain)

**Status:**
- Clients Blazor: ✅ ClientList + ClientEdit refactored
- TaxFilings Blazor: ⏳ Pending refactor (pages exist)
- Faqs: ⏳ API + Blazor pending
- Announcements: ⏳ API + Blazor pending
- Phase 6 SignalR: ⏳ Deferred

Next: Refactor TaxFilings Blazor pages, then Faqs & Announcements
Build: ✅ Success (0 errors, 2 warnings in Dashboard)

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

TaxBaik.Web/Controllers/TaxFilingController.cs

@@ -0,0 +1,84 @@
+using Microsoft.AspNetCore.Authorization;
+using Microsoft.AspNetCore.Mvc;
+using TaxBaik.Application.Services;
+using TaxBaik.Domain.Entities;
+
+namespace TaxBaik.Web.Controllers;
+
+[ApiController]
+[Route("api/[controller]")]
+[Authorize]
+public class TaxFilingController : ControllerBase
+{
+    private readonly TaxFilingService _taxFilingService;
+
+    public TaxFilingController(TaxFilingService taxFilingService)
+    {
+        _taxFilingService = taxFilingService;
+    }
+
+    [HttpGet("upcoming")]
+    public async Task<IActionResult> GetUpcoming([FromQuery] int daysAhead = 30)
+    {
+        var filings = await _taxFilingService.GetUpcomingAsync(daysAhead);
+        return Ok(new { data = filings });
+    }
+
+    [HttpGet("client/{clientId}")]
+    public async Task<IActionResult> GetByClientId(int clientId)
+    {
+        var filings = await _taxFilingService.GetByClientIdAsync(clientId);
+        return Ok(new { data = filings });
+    }
+
+    [HttpGet("{id}")]
+    public async Task<IActionResult> GetById(int id)
+    {
+        var filing = await _taxFilingService.GetByIdAsync(id);
+        if (filing == null)
+            return NotFound(new ProblemDetails { Title = "신고 일정을 찾을 수 없습니다.", Status = StatusCodes.Status404NotFound });
+
+        return Ok(filing);
+    }
+
+    [HttpPost]
+    public async Task<IActionResult> Create([FromBody] TaxFiling filing)
+    {
+        try
+        {
+            var filingId = await _taxFilingService.CreateAsync(filing);
+            var result = await _taxFilingService.GetByIdAsync(filingId);
+            return CreatedAtAction(nameof(GetById), new { id = filingId }, result);
+        }
+        catch (Exception ex)
+        {
+            return BadRequest(new ProblemDetails { Title = ex.Message, Status = StatusCodes.Status400BadRequest });
+        }
+    }
+
+    [HttpPut("{id}")]
+    public async Task<IActionResult> Update(int id, [FromBody] TaxFiling filing)
+    {
+        filing.Id = id;
+        try
+        {
+            await _taxFilingService.UpdateAsync(filing);
+            var result = await _taxFilingService.GetByIdAsync(id);
+            if (result == null)
+                return NotFound(new ProblemDetails { Title = "신고 일정을 찾을 수 없습니다.", Status = StatusCodes.Status404NotFound });
+
+            return Ok(result);
+        }
+        catch (Exception ex)
+        {
+            return BadRequest(new ProblemDetails { Title = ex.Message, Status = StatusCodes.Status400BadRequest });
+        }
+    }
+
+    [HttpDelete("{id}")]
+    public async Task<IActionResult> Delete(int id)
+    {
+        await _taxFilingService.DeleteAsync(id);
+        return NoContent();
+    }
+}