fix: Update connection string handling to allow for empty configuration

src/EchoHub.Server/Program.cs

@@ -39,8 +39,10 @@ while (true)
 
         // ── SQLite + EF Core ─────────────────────────────────────────────────
         var defaultDbPath = Path.Combine(AppContext.BaseDirectory, "echohub.db");
-        var connectionString = builder.Configuration.GetConnectionString("DefaultConnection")
-            ?? $"Data Source={defaultDbPath}";
+        var configured = builder.Configuration.GetConnectionString("DefaultConnection");
+        var connectionString = string.IsNullOrWhiteSpace(configured)
+            ? $"Data Source={defaultDbPath}"
+            : configured;
 
         builder.Services.AddDbContext<EchoHubDbContext>(options =>
             options.UseSqlite(connectionString));

src/EchoHub.Server/appsettings.example.json

@@ -1,7 +1,7 @@
 {
   "Urls": "http://0.0.0.0:5000",
   "ConnectionStrings": {
-    "DefaultConnection": "Data Source=echohub.db"
+    "DefaultConnection": ""
   },
   "Jwt": {
     "Secret": "CHANGE_ME_minimum_32_characters_long_secret_key",