Remove hardcoded secrets from appsettings.json

JWT signing key and database connection string were committed in plaintext.
Both are now empty placeholders; real values live in .NET User Secrets (dev)
and must be supplied via environment variables or a secrets manager in production.

Marechai.Server/appsettings.json

@@ -1,6 +1,6 @@
 {
   "ConnectionStrings": {
-    "DefaultConnection": "server=mariadb-dev.claunia.com;port=3306;user=marechai;password=marechaipass;database=marechai;TreatTinyAsBoolean=false"
+    "DefaultConnection": ""
   },
   "Logging":           {
     "LogLevel": {
@@ -67,7 +67,7 @@
     }
   ],
   "Jwt":             {
-    "Key":      "!SomethingSecret!!SomethingSecret!!SomethingSecret!!SomethingSecret!",
+    "Key":      "",
     "Issuer":   "apiWithAuthBackend",
     "Audience": "apiWithAuthBackend"
   },