WebAPI for database integrity added

Restructure the solution
* The project has been split into the database integration "src/webapi" and the Blazor wasm project "src/y4f".

WebAPI
* API with CRUD capabilities

* The main task of the "WebAPI" project is to access the database data and make it available to the Blazor-WASM project via an API.

* GET example of the allergy table via the file "TestFetchAllergienData.razor", which can be tested via the browser "/TestFetchAllergienData".

src/WebApi/Controllers/RabatteController.cs

@@ -0,0 +1,138 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading.Tasks;
+using Microsoft.AspNetCore.Http;
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.EntityFrameworkCore;
+using WebApi.Data;
+using WebApi.Models;
+
+namespace WebApi.Controllers
+{
+    [Route("api/[controller]")]
+    [ApiController]
+    public class RabatteController : ControllerBase
+    {
+        private readonly WebApiContext _context;
+
+        public RabatteController(WebApiContext context)
+        {
+            _context = context;
+        }
+
+        // GET: api/Rabatte
+        [HttpGet]
+        public async Task<ActionResult<IEnumerable<Rabatt>>> GetRabatts()
+        {
+          if (_context.Rabatts == null)
+          {
+              return NotFound();
+          }
+            return await _context.Rabatts.ToListAsync();
+        }
+
+        // GET: api/Rabatte/5
+        [HttpGet("{id}")]
+        public async Task<ActionResult<Rabatt>> GetRabatt(int id)
+        {
+          if (_context.Rabatts == null)
+          {
+              return NotFound();
+          }
+            var rabatt = await _context.Rabatts.FindAsync(id);
+
+            if (rabatt == null)
+            {
+                return NotFound();
+            }
+
+            return rabatt;
+        }
+
+        // PUT: api/Rabatte/5
+        // To protect from overposting attacks, see https://go.microsoft.com/fwlink/?linkid=2123754
+        [HttpPut("{id}")]
+        public async Task<IActionResult> PutRabatt(int id, Rabatt rabatt)
+        {
+            if (id != rabatt.Idrabatt)
+            {
+                return BadRequest();
+            }
+
+            _context.Entry(rabatt).State = EntityState.Modified;
+
+            try
+            {
+                await _context.SaveChangesAsync();
+            }
+            catch (DbUpdateConcurrencyException)
+            {
+                if (!RabattExists(id))
+                {
+                    return NotFound();
+                }
+                else
+                {
+                    throw;
+                }
+            }
+
+            return NoContent();
+        }
+
+        // POST: api/Rabatte
+        // To protect from overposting attacks, see https://go.microsoft.com/fwlink/?linkid=2123754
+        [HttpPost]
+        public async Task<ActionResult<Rabatt>> PostRabatt(Rabatt rabatt)
+        {
+          if (_context.Rabatts == null)
+          {
+              return Problem("Entity set 'WebApiContext.Rabatts'  is null.");
+          }
+            _context.Rabatts.Add(rabatt);
+            try
+            {
+                await _context.SaveChangesAsync();
+            }
+            catch (DbUpdateException)
+            {
+                if (RabattExists(rabatt.Idrabatt))
+                {
+                    return Conflict();
+                }
+                else
+                {
+                    throw;
+                }
+            }
+
+            return CreatedAtAction("GetRabatt", new { id = rabatt.Idrabatt }, rabatt);
+        }
+
+        // DELETE: api/Rabatte/5
+        [HttpDelete("{id}")]
+        public async Task<IActionResult> DeleteRabatt(int id)
+        {
+            if (_context.Rabatts == null)
+            {
+                return NotFound();
+            }
+            var rabatt = await _context.Rabatts.FindAsync(id);
+            if (rabatt == null)
+            {
+                return NotFound();
+            }
+
+            _context.Rabatts.Remove(rabatt);
+            await _context.SaveChangesAsync();
+
+            return NoContent();
+        }
+
+        private bool RabattExists(int id)
+        {
+            return (_context.Rabatts?.Any(e => e.Idrabatt == id)).GetValueOrDefault();
+        }
+    }
+}