Feature request: support loading FILTER_REGEX from a file path

[fdefilippo]

Describe the problem

Currently, FILTER_REGEX can only be set via environment variable. For long or frequently updated blacklists, this is inconvenient:

• Environment variables have length limits on some systems
• Updating the filter requires restarting the container with a modified compose file
• There's no easy way to manage the list outside the deployment config
• Long regex patterns clutter docker-compose.yml / systemd unit files

Describe the solution you'd like

Allow FILTER_REGEX to accept either:

1. A raw regex string (as it does now) — for simple, static filters
2. A file path prefix, e.g. /etc/remark42/filter.txt — the engine reads the regex pattern from the file

Suggestion: if the value starts with / or ./, treat it as a file path. Otherwise treat it as a regex string.

Alternative

Add a separate env var like FILTER_REGEX_FILE=/path/to/filter.txt that takes priority over FILTER_REGEX when set.

Use case example

# In docker-compose.yml (clean):
- FILTER_REGEX_FILE=/etc/remark42/blacklist.txt

# On the host (/etc/remark42/blacklist.txt):
\b(cazzo|merda|puttana|spam|viagra)\b|https?://\S*(?:casino|poker)\S*

This way the docker-compose.yml stays clean and the blacklist can be updated without touching the deployment config.