feat(logging): write warnings to stderr in prod as they happen (#524 follow-up)

[Neophytis]

Target branch: feature/docker-master (prod builds from it). Follow-up to #524 / #558. Config only; it survives the #519 upgrade.

Why

Since #558 (rox 3.8.1), prod writes only errors, each with the records buffered before it in the same request. Warnings from requests that don't fail are no longer written anywhere. On 3.8.0 that was about 28k a day, and several pointed at real problems:

• the avatar fallback from Backport fix for 548 to production source. #556 (Avatar image ... is missing or not readable), which says which members have unreadable avatar files
• Header may not contain more than a single header (Language switcher: broken redirect without Referer (Header may not contain more than a single header) and open redirect #550)
• session_start(): Failed to decode session object (Sessions that cannot be decoded return 500 (Failed to start the session) #532)
• Undefined array key in SuggestLocationModel and the Wikitext parser

What

config/packages/prod/monolog.yaml: a warnings filter handler that passes only the warning level, straight to php://stderr as JSON, without a buffer.

• Errors stay with main, so an error is never written twice.
• A warning in a request that later fails appears twice: once on its own, once as context of the error. That's about 400 requests a day.
• Notices, debug and deprecations stay off.

Volume: about 28k lines a day, about 20k once ROX-502 (the alias.cache.ini warning, about 7.5k a day) is fixed. All of VictoriaLogs takes in about 876k rows a day.

Tested

In the rox:3.8.1 image with this config mounted, using a test kernel that exposes the loggers:

Case	stderr
normal request (security.DEBUG + app.INFO)	nothing
404	nothing
warning only	1 line (php.WARNING)
notice	nothing
500	3 lines, CRITICAL once
warning, then 500 in the same request	WARNING twice (alone + context), CRITICAL once
deprecation (DEPRECATION_LOG unset)	nothing

var/log stayed empty, and debug:config monolog resolves.

After merge (stage)

• service:php level:warn* fields.channel:* shows warnings in VictoriaLogs.
• Volume per day: _time:24h env:stage service:php fields.level_name:WARNING | stats count().