Add CacheController and caching support for signatures

Introduced a new `CacheController` to manage cached data for
receiver signatures using distributed caching. Added endpoints
to save, retrieve, and delete cached signatures.

Created a `SignatureCacheRequest` model for caching payloads
and a `CacheOptions` class to configure cache settings,
including optional expiration. Updated `Program.cs` to bind
`CacheOptions` to the `Cache` section in `appsettings.json`.

Added a new `Cache` section in `appsettings.json` with a
`SignatureCacheExpiration` property, defaulting to `null`
(no expiration).

EnvelopeGenerator.API/Controllers/CacheController.cs

@@ -0,0 +1,82 @@
+using Microsoft.AspNetCore.Authorization;
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.Extensions.Caching.Distributed;
+using Microsoft.Extensions.Options;
+using System.Text.Json;
+using EnvelopeGenerator.API.Options;
+using EnvelopeGenerator.Domain.Constants;
+
+namespace EnvelopeGenerator.API.Controllers;
+
+/// <summary>
+/// Manages cached data for receivers using distributed cache.
+/// </summary>
+[ApiController]
+[Route("api/[controller]")]
+[Authorize(Policy = AuthPolicy.Receiver)]
+public class CacheController(
+    IDistributedCache cache,
+    IOptions<CacheOptions> cacheOptions) : ControllerBase
+{
+    private const string SignatureCacheKeyPrefix = "signature:91751687-8ae6-4777-bf5f-b8846085e62e:";
+
+    /// <summary>
+    /// Stores a receiver's signature in cache for the specified envelope.
+    /// </summary>
+    [HttpPost("SignatureCapture/{envelopeKey}")]
+    public async Task<IActionResult> SaveSignature(
+        string envelopeKey,
+        [FromBody] SignatureCacheRequest request,
+        CancellationToken cancel)
+    {
+        var cacheKey = $"{SignatureCacheKeyPrefix}{envelopeKey}";
+        var json = JsonSerializer.Serialize(request);
+        
+        var options = cacheOptions.Value.SignatureCacheExpiration.HasValue
+            ? new DistributedCacheEntryOptions { AbsoluteExpirationRelativeToNow = cacheOptions.Value.SignatureCacheExpiration.Value }
+            : null;
+
+        await cache.SetStringAsync(cacheKey, json, options ?? new DistributedCacheEntryOptions(), cancel);
+
+        return Ok();
+    }
+
+    /// <summary>
+    /// Retrieves a cached signature for the specified envelope.
+    /// </summary>
+    [HttpGet("SignatureCapture/{envelopeKey}")]
+    public async Task<IActionResult> GetSignature(string envelopeKey, CancellationToken cancel)
+    {
+        var cacheKey = $"{SignatureCacheKeyPrefix}{envelopeKey}";
+        var json = await cache.GetStringAsync(cacheKey, cancel);
+
+        if (json is null)
+            return NotFound();
+
+        var signature = JsonSerializer.Deserialize<SignatureCacheRequest>(json);
+        return Ok(signature);
+    }
+
+    /// <summary>
+    /// Deletes a cached signature for the specified envelope.
+    /// </summary>
+    [HttpDelete("SignatureCapture/{envelopeKey}")]
+    public async Task<IActionResult> DeleteSignature(string envelopeKey, CancellationToken cancel)
+    {
+        var cacheKey = $"{SignatureCacheKeyPrefix}{envelopeKey}";
+        await cache.RemoveAsync(cacheKey, cancel);
+
+        return Ok();
+    }
+}
+
+/// <summary>
+/// Request model for caching signature data.
+/// </summary>
+public sealed record SignatureCacheRequest(
+    string? DataUrl,
+    string? FullName,
+    string? Position,
+    string? Place);
+
+