Fix HAProxy duplicate server names in backend configuration

PR fly-apps#297 introduced a bug by using the same prefix "pg" for both
server-template lines, causing duplicate server names (pg1-pg10).

Changed second template to use "alt" prefix for alternative region servers:
- server-template pg: Primary region servers (pg1-pg10)
- server-template alt: Alternative region servers (alt1-alt10)

This maintains the performance optimization of checking PRIMARY_REGION
first while fixing the "another server named 'pgX' was already defined" error.

Fixes PITR restore failures where HAProxy fails to start.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: jcrapuchettes

config/haproxy.cfg

@@ -35,5 +35,5 @@ backend bk_db
     http-check expect string primary
     http-check disable-on-404
     server-template pg 10 $PRIMARY_REGION.$FLY_APP_NAME.internal:5433 check port 5500 resolvers flydns resolve-prefer ipv6 init-addr none on-marked-down shutdown-sessions
-    server-template pg 10 $FLY_APP_NAME.internal:5433 check port 5500 resolvers flydns resolve-prefer ipv6 init-addr none on-marked-down shutdown-sessions
+    server-template alt 10 $FLY_APP_NAME.internal:5433 check port 5500 resolvers flydns resolve-prefer ipv6 init-addr none on-marked-down shutdown-sessions
     server pg [$PG_LISTEN_ADDRESS]:5433 check backup port 5500 on-marked-down shutdown-sessions