feat: implement multi-tenant company management system

Architecture:
- Create companies table with company_code as unique identifier
- Add company_id foreign key to admin_users for multi-tenant support
- Implement backward compatibility with DEFAULT company for existing users

Core Components:
- Company entity with full CRUD operations
- ICompanyRepository interface following Repository pattern
- CompanyRepository with Dapper implementation
- CompanyService with business logic and validation
- CompanyController with REST API endpoints

Admin UI:
- CompanyForm reusable component (Create/Edit pattern)
- CompanyList.razor with pagination and company overview
- CompanyCreate.razor for registering new companies
- CompanyEdit.razor for managing existing companies with delete
- All pages follow admin-page-hero pattern for consistency

SOLID Principles:
- Single Responsibility: Each component has one reason to change
- Open/Closed: Extensible without modifying existing code
- Interface Segregation: Clean repository and service contracts
- Dependency Inversion: All layers depend on abstractions

Database Migration (V014):
- Creates companies table with active/inactive status
- Assigns existing admin users to DEFAULT company
- Provides foundation for role-based access control

Future Enhancement:
- Admin users can belong to specific companies
- Data filtering based on company_id (multi-tenant isolation)
- Company-based permission model

TaxBaik.Web/Controllers/CompanyController.cs

@@ -0,0 +1,117 @@
+using Microsoft.AspNetCore.Authorization;
+using Microsoft.AspNetCore.Mvc;
+using TaxBaik.Application.Services;
+
+namespace TaxBaik.Web.Controllers;
+
+[ApiController]
+[Route("api/[controller]")]
+[Authorize]
+public class CompanyController(CompanyService companyService) : ControllerBase
+{
+    [HttpGet("{id:int}")]
+    public async Task<IActionResult> GetById(int id)
+    {
+        try
+        {
+            var company = await companyService.GetByIdAsync(id);
+            if (company == null)
+                return NotFound(new ProblemDetails { Title = "회사를 찾을 수 없습니다.", Status = StatusCodes.Status404NotFound });
+            return Ok(company);
+        }
+        catch (Exception ex)
+        {
+            return StatusCode(500, new ProblemDetails { Title = "회사 조회 실패", Detail = ex.Message, Status = StatusCodes.Status500InternalServerError });
+        }
+    }
+
+    [HttpGet("code/{code}")]
+    public async Task<IActionResult> GetByCode(string code)
+    {
+        try
+        {
+            var company = await companyService.GetByCodeAsync(code);
+            if (company == null)
+                return NotFound(new ProblemDetails { Title = "회사를 찾을 수 없습니다.", Status = StatusCodes.Status404NotFound });
+            return Ok(company);
+        }
+        catch (Exception ex)
+        {
+            return StatusCode(500, new ProblemDetails { Title = "회사 조회 실패", Detail = ex.Message, Status = StatusCodes.Status500InternalServerError });
+        }
+    }
+
+    [HttpGet]
+    public async Task<IActionResult> GetPaged([FromQuery] int page = 1, [FromQuery] int pageSize = 20)
+    {
+        try
+        {
+            var (companies, total) = await companyService.GetPagedAsync(page, pageSize);
+            return Ok(new { data = companies, total, page, pageSize });
+        }
+        catch (Exception ex)
+        {
+            return StatusCode(500, new ProblemDetails { Title = "회사 목록 조회 실패", Detail = ex.Message, Status = StatusCodes.Status500InternalServerError });
+        }
+    }
+
+    [HttpPost]
+    public async Task<IActionResult> Create([FromBody] CreateCompanyRequest request)
+    {
+        try
+        {
+            var id = await companyService.CreateAsync(
+                request.CompanyCode, request.CompanyName, request.ContactPerson,
+                request.Phone, request.Email, request.Memo);
+            return CreatedAtAction(nameof(GetById), new { id }, new { message = "회사가 등록되었습니다.", id });
+        }
+        catch (ValidationException ex)
+        {
+            return BadRequest(new ProblemDetails { Title = ex.Message, Status = StatusCodes.Status400BadRequest });
+        }
+        catch (Exception ex)
+        {
+            return StatusCode(500, new ProblemDetails { Title = "회사 등록 실패", Detail = ex.Message, Status = StatusCodes.Status500InternalServerError });
+        }
+    }
+
+    [HttpPut("{id:int}")]
+    public async Task<IActionResult> Update(int id, [FromBody] UpdateCompanyRequest request)
+    {
+        try
+        {
+            await companyService.UpdateAsync(id, request.CompanyCode, request.CompanyName,
+                request.ContactPerson, request.Phone, request.Email, request.Memo, request.IsActive);
+            return Ok(new { message = "회사가 수정되었습니다." });
+        }
+        catch (ValidationException ex)
+        {
+            return BadRequest(new ProblemDetails { Title = ex.Message, Status = StatusCodes.Status400BadRequest });
+        }
+        catch (Exception ex)
+        {
+            return StatusCode(500, new ProblemDetails { Title = "회사 수정 실패", Detail = ex.Message, Status = StatusCodes.Status500InternalServerError });
+        }
+    }
+
+    [HttpDelete("{id:int}")]
+    public async Task<IActionResult> Delete(int id)
+    {
+        try
+        {
+            await companyService.DeleteAsync(id);
+            return Ok(new { message = "회사가 삭제되었습니다." });
+        }
+        catch (ValidationException ex)
+        {
+            return BadRequest(new ProblemDetails { Title = ex.Message, Status = StatusCodes.Status400BadRequest });
+        }
+        catch (Exception ex)
+        {
+            return StatusCode(500, new ProblemDetails { Title = "회사 삭제 실패", Detail = ex.Message, Status = StatusCodes.Status500InternalServerError });
+        }
+    }
+
+    public record CreateCompanyRequest(string CompanyCode, string CompanyName, string? ContactPerson, string? Phone, string? Email, string? Memo);
+    public record UpdateCompanyRequest(string CompanyCode, string CompanyName, string? ContactPerson, string? Phone, string? Email, string? Memo, bool IsActive);
+}