bug(config): two parallel env var names for browser host — AUTOBOT_BROWSER_SERVICE_HOST vs AUTOBOT_BROWSER_HOST can silently diverge

[mrveiss]

Problem

The browser VM host is configured via two separate env var names that are read by different code paths:

Var	Read by	Set by
AUTOBOT_BROWSER_SERVICE_HOST	playwright_service.py:465 (PlaywrightService)	backend.env.j2 (fixed in #3728)
AUTOBOT_BROWSER_HOST	service_config.py:77 (ServiceConfigMixin.get_host("browser"))	backend.env.j2 (fixed in #3728)

Both are now written by Ansible from the same source value, but they can drift if one is changed manually or if a future change only updates one.

ServiceConfigMixin.get_host() checks AUTOBOT_{SERVICE}_HOST (i.e. AUTOBOT_BROWSER_HOST) while PlaywrightService hardcodes AUTOBOT_BROWSER_SERVICE_HOST. These are the same physical VM.

Fix

Consolidate to one env var. PlaywrightService should use os.getenv("AUTOBOT_BROWSER_HOST") (the pattern used by all other services in ServiceConfigMixin), removing the special-cased AUTOBOT_BROWSER_SERVICE_HOST name. Update backend.env.j2 to only emit one var.