Add Steam Workshop mod auto-updates

[SemHiel2003]

Closes #215.

This adds Steam Workshop mod updates to the existing mods updater:

• discover direct child mod directories from their meta.cpp publishedid
• query Steam's GetPublishedFileDetails endpoint and compare time_updated with the version cached in mods.yml
• use Steam's Workshop consumer_app_id, with a numeric server-updater.software value as fallback
• download through SteamCMD and honor the existing NOTIFY, MANUAL, and AUTOMATIC profiles
• copy successful automatic updates into the existing mod directory and cache the checked version

No new dependency is added. The Steam endpoint requires a POST form request, so the implementation uses HttpURLConnection and the Gson dependency already in the project.

Derived work and changes from #290

This contribution builds on the inactive, closed PR #290, which itself builds on #280, as permitted by the derived-PR terms in BOUNTY_TERMS.md.

On top of that implementation, this version:

• does not treat the game-specific meta.cpp timestamp as comparable to Steam's Unix time_updated
• resolves the owning Workshop app from consumer_app_id (for the issue's DayZ example, 221100, rather than dedicated-server app 223350)
• adds connection/read timeouts to the Steam metadata request
• requires both a SteamCMD success line and the expected downloaded directory before reporting success

Validation

• mvn -q -DskipTests compile
• mvn -q -Dtest='SteamWorkshopModTest,SteamCMDTest#buildsWorkshopItemCommand' test — 5/5 passed
• git diff --check
• a read-only call to Steam's public API returned result=1 and consumer_app_id=221100 for Workshop item 1559212036

The full local package run passed 32 tests, including all new tests. The existing network-dependent SystemCheckerTest produced one environment error because www.google.com was unreachable.

AI assistance and human validation

This contribution was developed predominantly with AI assistance. The human account owner read a plain-language review of the behavior and limitations, personally ran the compile and focused test suite against commit 3d2041c43a888bfdb3831ad2713991efc60f8868, and confirmed zero failures and zero errors before this PR was opened. I understand the change and take responsibility for its behavior and quality.

The PayPal identity-verification requirement will be satisfied before merge without putting payment details in this PR discussion.