The verification-service and testagent-transform clients both call
`GET /api/v1/log/entries/{content_hash}` to read a stored envelope and
inclusion proof back out of the log, but the server previously only
implemented the POST (write) half of ADR-0005. Add:
- `Backend.Get` plus `backend.Entry` and `ErrEntryNotFound` so the
memory backend can serve the read; Trillian stays stubbed and
continues returning 503 (`ErrTrillianUnavailable`) until a later
subtask wires the real read path.
- `GET /api/v1/log/entries/{hash}` on the HTTP server, routing 404 /
503 / 500 with the same error envelope as the POST half.
- `GET /healthz` so the Phase-1 smoke-test harness can wait for the
service to come up instead of racing against its bind.
Co-Authored-By: Paperclip <noreply@paperclip.ing>
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