WPB-19713: Implement channels and channelsCount in user-groups endpoints

[blackheaven]

https://wearezeta.atlassian.net/browse/WPB-19713

Checklist

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

WIP

• Rely on persistence (17126)
• Tests

Design considerations

I'm not happy with the design, I can either:

• Leave is as it
• Push the fetch (and options) in the interpreter, making it rely on GalleyAPIAccess
• Use/Introduce a data-type to enforce channels/channelsCount from the effect

[Copilot]

Pull Request Overview

This PR implements the channels and channelsCount fields in user-groups endpoints, addressing ticket WPB-19713. The implementation adds functionality to fetch and include channel information when retrieving user groups.

Key changes:

• Add ListChannels operation to retrieve channels associated with user groups
• Implement conditional channel fetching based on include_channels query parameter
• Update API routes and test coverage for the new functionality

Reviewed Changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
services/brig/src/Brig/API/Public.hs	Implements channel fetching logic in getUserGroup and getUserGroups handlers
services/brig/default.nix	Adds vector dependency for channel count calculations
services/brig/brig.cabal	Adds vector library dependency
postgres-schema.sql	Contains duplicate table definition (appears to be unintentional)
libs/wire-subsystems/test/unit/Wire/MockInterpreters/UserGroupStore.hs	Implements mock interpreter for ListUserGroupChannels
libs/wire-subsystems/src/Wire/UserGroupSubsystem/Interpreter.hs	Adds listChannels function with proper authorization checks
libs/wire-subsystems/src/Wire/UserGroupSubsystem.hs	Defines ListChannels operation in the subsystem interface
libs/wire-subsystems/src/Wire/UserGroupStore/Postgres.hs	Implements PostgreSQL query for listing user group channels
libs/wire-subsystems/src/Wire/UserGroupStore.hs	Adds ListUserGroupChannels to the store interface
libs/wire-api/src/Wire/API/Routes/Public/Brig.hs	Updates API documentation and adds include_channels query parameter
integration/test/Test/UserGroup.hs	Updates test to verify channel inclusion functionality
integration/test/API/Brig.hs	Adds helper function for testing channel inclusion
changelog.d/2-features/WPB-19713	Documents the feature addition

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