feat: extract secrets to .env, remove hardcoded credentials

- Backend: .env file for DB/JWT/Redis/MinIO config, appsettings.json cleared
- Backend: Program.cs loads .env at startup (no extra NuGet packages)
- Frontend: .env files for VITE_API_URL, api-clients use import.meta.env
- Added vite-env.d.ts type declarations for both frontends
- All hardcoded localhost:5000 replaced with env variable
- Added .env.example template for onboarding

frontend-patient/src/vite-env.d.ts

@@ -0,0 +1,9 @@
+/// <reference types="vite/client" />
+
+interface ImportMetaEnv {
+  readonly VITE_API_URL: string;
+}
+
+interface ImportMeta {
+  readonly env: ImportMetaEnv;
+}