fix(fail2ban): improve handling of fail2ban

[denis-ev]

• Still needs a long time test, as my previous PR worked until it didn't
• I'll leave this open as Draft so others have the opportunity to test, I'm currently running the change on my system and will check regularly if I have any negative counters with unbanned IPs.

Contribution Guidelines

• I've read the contribution guidelines and wholeheartedly agree them

What does this PR include?

Fixes #5879

Short Description

Fixes fail2ban/netfilter so that bans are actually removed when they expire or when the user clicks unban. Previously, unban() returned early when the banned network was not in the in-memory bans dict: it only cleared F2B_QUEUE_UNBAN and did not remove the entry from F2B_ACTIVE_BANS or remove the firewall rule. That left IPs stuck in Redis and in the firewall, the UI timer went negative, and the unban button had no effect until the netfilter container was restarted.

This change makes unban always perform Redis cleanup and firewall unban when called (by autopurge on expiry or by the unban queue). The in-memory bans dict is only used for bookkeeping (attempts, ban_counter). Invalid or unparseable network strings are handled by still removing the entry from Redis and only skipping the firewall unban, so stale entries do not remain in the UI.

Affected Containers

• netfilter-mailcow

Did you run tests?

What did you tested?

• Triggered multiple failed admin UI logins until an IP was banned.
• Waited for the ban timer to expire (and, in another run, used the unban button before expiry).
• Confirmed that the IP was removed from the active ban list and that connectivity was restored without restarting netfilter.
• Confirmed that the unban button works for both recently banned IPs and for entries that had already expired (negative timer).

What were the final results? (Awaited, got)

So far IPs are getting unbanned as expected, needs a bit of a longer runtime to properly check, as the previous PR only failed after a while and not instantly

[milkmaker]

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