Standardize English language usage and improve environment configuration

Replaced German text with English across the frontend UI for consistency and accessibility. Enhanced clarity in `.env.example` and `README.md`, adding detailed comments for environment variables and prerequisites. Improved documentation for setup, security, and troubleshooting.

ops/.env.example

@@ -1,27 +1,57 @@
-# App
+# ------------------------------
+# Application
+# ------------------------------
+# Display name used in API docs/UI.
 APP_NAME=NexaPG Monitor
+# Runtime environment: dev | staging | prod | test
 ENVIRONMENT=dev
+# Backend log level: DEBUG | INFO | WARNING | ERROR
 LOG_LEVEL=INFO
 
-# Core DB
+# ------------------------------
+# Core Database (internal metadata DB)
+# ------------------------------
+# Database that stores users, targets, metrics, query stats, and audit logs.
 DB_NAME=nexapg
 DB_USER=nexapg
 DB_PASSWORD=nexapg
+# Host port mapped to the internal PostgreSQL container port 5432.
 DB_PORT=5433
 
-# Backend
+# ------------------------------
+# Backend API
+# ------------------------------
+# Host port mapped to backend container port 8000.
 BACKEND_PORT=8000
+# JWT signing secret. Change this in every non-local environment.
 JWT_SECRET_KEY=change_this_super_secret
 JWT_ALGORITHM=HS256
+# Access token lifetime in minutes.
 JWT_ACCESS_TOKEN_MINUTES=15
+# Refresh token lifetime in minutes (10080 = 7 days).
 JWT_REFRESH_TOKEN_MINUTES=10080
-# Generate with: python -c "from cryptography.fernet import Fernet; print(Fernet.generate_key().decode())"
+# Key used to encrypt monitored target passwords at rest.
+# Generate with:
+# python -c "from cryptography.fernet import Fernet; print(Fernet.generate_key().decode())"
 ENCRYPTION_KEY=REPLACE_WITH_FERNET_KEY
+# Allowed CORS origins for browser clients.
+# Use comma-separated values, e.g.:
+# CORS_ORIGINS=http://localhost:5173,https://nexapg.example.com
+# Dev-only shortcut:
+# CORS_ORIGINS=*
 CORS_ORIGINS=http://localhost:5173,http://localhost:8080
+# Target polling interval in seconds.
 POLL_INTERVAL_SECONDS=30
+# Initial admin bootstrap user (created on first startup if not present).
 INIT_ADMIN_EMAIL=admin@example.com
 INIT_ADMIN_PASSWORD=ChangeMe123!
 
+# ------------------------------
 # Frontend
+# ------------------------------
+# Host port mapped to frontend container port 80.
 FRONTEND_PORT=5173
-VITE_API_URL=http://localhost:8000/api/v1
+# Base API URL used at frontend build time.
+# For reverse proxy + SSL, keep this relative to avoid mixed-content issues.
+# Example direct mode: VITE_API_URL=http://localhost:8000/api/v1
+VITE_API_URL=/api/v1