Fix peer IP allocation: fill subnet gaps, sort peers by IP, backup config

[leonidorlov-hash]

Problem

_get_next_ip() in managers/awg_manager.py and managers/wireguard_manager.py took the last IP in config file order and incremented it. This caused:

1. Duplicate IPs — if peers in awg0.conf/wg0.conf are not sorted ascending (after manual edits, migrations, deletions), the next client could get an AllowedIPs already used by another peer. WireGuard then applies only the last matching [Peer], breaking one of the clients.
2. Gaps never reused — deleting a client (e.g. 10.8.1.5) left the address unused forever; new clients kept getting higher IPs.
3. Octet overflow — awg_manager produced invalid octets (254 → 257, 255 → 257).

Fix

• _get_next_ip() now collects all used addresses into a set and returns the first free IP in the subnet (.2–.254), filling gaps. Raises a clear error when the subnet is exhausted.
• New peers are no longer blindly appended (>> config). A new helper _insert_peer_sorted() rewrites the config with all [Peer] sections sorted by AllowedIPs, keeping the [Interface] header intact.
• Before overwriting, a timestamped backup (awg0.conf.bak.YYYYMMDD_HHMMSS) is created inside the container.

Notes

• Existing duplicate peers are not touched — this only fixes allocation for new clients.
• Tested locally: config reassembly preserves the interface section and sorts peers numerically by IP.