admin-api: Pocket-ID client_credentials TokenSource with OIDC discovery + cache (replaces StaticToken in prod)
* services/admin-api/internal/attest/pocket_id_token_source.go — attest.TokenSource impl: OIDC discovery via /.well-known/openid-configuration, HTTP Basic client_credentials grant, mutex-guarded in-memory token cache with configurable RefreshLeeway (default 30s = "verwirft bei exp-30s" from DEV-602).
* services/admin-api/internal/attest/pocket_id_token_source_test.go — 10 subtests against an httptest OIDC mock: happy path, cache hold, exp-30s refresh boundary, optional scope/audience, bad creds (401), missing config, discovery failures, non-positive expires_in, type-conformance.
* services/admin-api/cmd/admin-api/main.go — buildTokenSource(): -bearer flag wins (dev), else -oidc-issuer + POCKET_ID_CLIENT_ID + POCKET_ID_CLIENT_SECRET env → PocketIDTokenSource (prod); hard-fail at startup if issuer is set but credentials are missing. New flags -pocket-id-scope / -pocket-id-audience for the client_credentials request.
* services/admin-api/README.md — split inbound/outbound Pocket-ID auth, document the admin-api OIDC client registration (UI + API), new flags, env vars, prod bootstrap example.
36/36 go tests pass, go vet clean.
Co-Authored-By: Paperclip <noreply@paperclip.ing>
be3ac9cdb8