feat(06-02): add environment docs and backup script

- .env.example documents all configuration options
- backup.sh creates timestamped archive of Docker volume
- Supports custom volume name via VOLUME_NAME env var

.env.example

@@ -0,0 +1,54 @@
+# TaskPlaner Environment Configuration
+# Copy to .env and customize for your deployment
+
+# ============================================
+# Server Configuration
+# ============================================
+
+# Port the server listens on (inside container)
+# Map to host port via docker-compose ports setting
+PORT=3000
+
+# ============================================
+# Data Storage
+# ============================================
+
+# Directory for database and uploads
+# Docker: /app/data (must match volume mount)
+# Local development: ./data
+TASKPLANER_DATA_DIR=/app/data
+
+# ============================================
+# Production URL (REQUIRED for production)
+# ============================================
+
+# The full URL where users access the app
+# Used for CSRF validation and generating absolute URLs
+# Example: https://tasks.example.com
+ORIGIN=http://localhost:3000
+
+# ============================================
+# Request Limits
+# ============================================
+
+# Maximum request body size
+# Supports K, M, G suffixes
+# Default: 512kb, recommended for images: 10M
+BODY_SIZE_LIMIT=10M
+
+# ============================================
+# Reverse Proxy Configuration
+# Uncomment when running behind nginx/traefik/etc
+# ============================================
+
+# Header containing original protocol (http/https)
+# PROTOCOL_HEADER=x-forwarded-proto
+
+# Header containing original host
+# HOST_HEADER=x-forwarded-host
+
+# Header containing original client IP
+# ADDRESS_HEADER=x-forwarded-for
+
+# Number of trusted proxies in front of app
+# XFF_DEPTH=1