fix: keep stored metadata when a project, group or user update does not set it #2030

Problem

When a project, group or user is updated, the whole metadata column is replaced with what the request sends. If a client leaves metadata out, the stored keys are wiped. #2019 fixed this for organizations. The other entities still have the bug.

Where it happens

• Project: UpdateByID and UpdateByName in internal/store/postgres/project_repository.go, called from UpdateProject.
• Group: UpdateByID in internal/store/postgres/group_repository.go, called from UpdateGroup.
• User: UpdateByID and UpdateByEmail in internal/store/postgres/user_repository.go, called from UpdateUser and UpdateCurrentUser.
• User UpdateByName is different. It skips metadata when the map is empty, so metadata can't be cleared on purpose.

Expected behavior

Same as organizations after #2019:

• If metadata is not set in the request, the stored metadata stays and schema validation is skipped.
• If metadata is set, it replaces the stored value. An empty {} clears it.

Tests

• Handler, repository and e2e tests for three cases: not set, replaced, cleared.

Out of scope

• Title and avatar are also replaced when omitted. That needs its own fix.