docs: add ENCRYPTION_KEY to .env.example for breaking change preparation

.env.example

@@ -1,5 +1,17 @@
 # See the documentation for more information: https://pocket-id.org/docs/configuration/environment-variables
+
+# These variables must be configured for your deployment:
 APP_URL=https://your-pocket-id-domain.com
+
+# Encryption key (choose one method):
+# Method 1: Direct key (simple but less secure)
+# Generate with: openssl rand -base64 32
+ENCRYPTION_KEY=
+# Method 2: File-based key (recommended)
+# Put the base64 key in a file and point to it here.
+# ENCRYPTION_KEY_FILE=/path/to/encryption_key
+
+# These variables are optional but recommended to review:
 TRUST_PROXY=false
 MAXMIND_LICENSE_KEY=
 PUID=1000