WPB-25919: Added a team feature flag for background effects

[blackheaven]

https://wearezeta.atlassian.net/browse/WPB-25919

Checklist

• Add a new entry in an appropriate subdirectory of changelog.d
• Read and follow the PR guidelines

[battermann]

LGTM

[Copilot]

Pull request overview

Adds a new team-level feature flag (backgroundEffects) to Wire Server’s feature-flag system so background effects availability in meetings can be configured (including lock/unlock behavior) via existing team feature flag APIs, with accompanying docs and integration coverage.

Changes:

• Introduces BackgroundEffectsConfig as a new IsFeatureConfig (default: enabled + unlocked) and wires it into the global Features list and defaults parsing.
• Exposes the flag through public/internal Galley APIs and Stern routes (get/set + lock/unlock).
• Extends integration coverage (feature-flag tests and migration expectations) and updates developer documentation + changelog.

Reviewed changes

Copilot reviewed 17 out of 17 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tools/stern/test/integration/API.hs	Adds Stern integration tests for backgroundEffects feature status + lock/unlock endpoints.
tools/stern/src/Stern/API/Routes.hs	Extends Stern API type with GET/PUT + lock/unlock routes for BackgroundEffectsConfig.
tools/stern/src/Stern/API.hs	Wires Stern handlers for the new feature routes.
services/galley/src/Galley/API/Teams/Features.hs	Adds SetFeatureConfig BackgroundEffectsConfig so teams can set the feature config.
services/galley/src/Galley/API/Public/Feature.hs	Exposes the new feature via the public feature API composition.
services/galley/src/Galley/API/Internal.hs	Includes background effects in internal lock-status update endpoints and the “all features” API composition.
libs/wire-subsystems/src/Wire/FeaturesConfigSubsystem/Types.hs	Adds GetFeatureConfig BackgroundEffectsConfig for subsystem-level retrieval.
libs/wire-api/src/Wire/API/Team/FeatureFlags.hs	Adds defaults parsing/serialization for backgroundEffects in server featureFlags config.
libs/wire-api/src/Wire/API/Team/Feature.hs	Defines BackgroundEffectsConfig, adds singleton constructor, exports it, and appends it to the Features type list.
libs/wire-api/src/Wire/API/Routes/Public/Galley/Feature.hs	Adds BackgroundEffectsConfig to the public feature routes type.
libs/wire-api/src/Wire/API/Routes/Internal/Galley.hs	Adds BackgroundEffectsConfig to internal legacy lock-status route list.
integration/test/Test/Migration/TeamFeatures.hs	Updates migration expectations to include backgroundEffects as an unlockable feature.
integration/test/Test/FeatureFlags/Util.hs	Adds default expected JSON for backgroundEffects and marks it as having explicit lock status.
integration/test/Test/FeatureFlags/BackgroundEffects.hs	New integration tests for patching and updating backgroundEffects.
integration/integration.cabal	Registers the new Test.FeatureFlags.BackgroundEffects module for compilation.
docs/src/developer/reference/config-options.md	Documents configuration syntax and internal/public API usage for backgroundEffects.
changelog.d/2-features/WPB-25919	Adds changelog entry for the new team feature flag.

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