perf: use stable sha256 cache keys for summarizer requests

JobTrackerApi/Services/SummarizerService.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Diagnostics;
 using System.Net.Http;
+using System.Security.Cryptography;
 using System.Text;
 using System.Text.Json;
 using System.Threading;
@@ -63,11 +64,20 @@ namespace JobTrackerApi.Services
             _cache = cache;
         }
 
+        private static string BuildCacheKey(string text, int maxLength, int minLength)
+        {
+            var payload = $"{text}\n::{maxLength}:{minLength}";
+            var hash = Convert.ToHexString(SHA256.HashData(Encoding.UTF8.GetBytes(payload))).ToLowerInvariant();
+            return $"summ:{hash}";
+        }
+
         public async Task<string?> SummarizeAsync(string text, int maxLength = 150, int minLength = 30)
         {
             if (string.IsNullOrWhiteSpace(text)) return null;
 
-            var key = $"summ:{text.GetHashCode()}:{maxLength}:{minLength}";
+            // Use a deterministic content hash instead of string.GetHashCode() so cache keys
+            // are collision-resistant and stable across process restarts.
+            var key = BuildCacheKey(text, maxLength, minLength);
             Interlocked.Increment(ref _requests);
 
             if (_cache.TryGetValue<string>(key, out var cached))