api: list repository sources

[ousamabenyounes]

Summary

• Add GET /api/repository/sources.
• Return distinct snapshot source triplets as {type, origin, directory} entries.
• Keep the response shape consistent with other repository list endpoints: total plus items.

Q	A
Issues	Fix #1223
Tests	go test ./api -run TestRepositorySourcesDeduplicatesSnapshotSources -count=1; go test ./api -count=1; ./run-ci.sh

Test verification (RED -> GREEN)

RED on upstream-base behavior with only the new test:

--- FAIL: TestRepositorySourcesDeduplicatesSnapshotSources (0.25s)
    api_repository_test.go:272:
        Error:       Not equal:
                    expected: 200
                    actual  : 404
        Messages:    body={"error":{"code":"not-found","message":"API endpoint not found"}}
FAIL
FAIL    github.com/PlakarKorp/plakar/api  0.271s

GREEN after the implementation:

ok      github.com/PlakarKorp/plakar/api  0.314s
ok      github.com/PlakarKorp/plakar/api  12.570s

Local CI replay on the final commit:

ok      github.com/PlakarKorp/plakar/api  15.298s  coverage: 73.3% of statements
ok      github.com/PlakarKorp/plakar/utils        0.026s  coverage: 71.4% of statements

The local CI runner also completed go build -v ./..., the full go test -p 4 -coverprofile=coverage.out -covermode=atomic ./..., and GOOS=windows GOARCH=amd64 go build -v . successfully.