feat: make screenshot uploads generally available

[wescopeland]

Removes the FEATURE_GAME_SCREENSHOT_UPLOADS feature flag and makes screenshot uploads available without having beta features enabled.

If we need a kill switch in the future for whatever reason, we can set SCREENSHOT_MAX_PENDING_SUBMISSIONS_PER_USER to 0.

[greptile-apps]

Greptile Summary

Screenshot uploads are generally available, with zero as the kill switch.

• Removes feature and beta gates.
• Denies uploads when the pending limit is nonpositive.
• Updates configuration and tests.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains.

_{Reviews (2): Last reviewed commit: "fix: address feedback" | Re-trigger Greptile}