feat: Add pool sharing for domains

Allow multiple domains to share the same backend pool using share_with parameter.
This saves pool slots when domains point to the same servers.

- Add share_with parameter to haproxy_add_domain
- Add helper functions for shared domain management
- Protect shared pools from being cleared on domain removal
- Update documentation with pool sharing examples

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

conf/wildcards.map

@@ -3,7 +3,8 @@
 # Uses map_dom for suffix matching
 
 .actions.it.com pool_3
-.anvil.it.com pool_4
+.anvil.it.com pool_9
 .bench.inouter.com pool_5
 .mcp.inouter.com pool_2
 .nas.inouter.com pool_1
+.ssh.inouter.com pool_11