spp_api_v2: bundle entry ETags use float seconds, not the microsecond versionId used elsewhere #564

Problem

Batch/transaction bundle entries return "etag": str(record.write_date.timestamp()) — float seconds
(spp_api_v2/services/bundle_service.py, ~320-474). Every resource's meta.versionId / ETag header is integer
microseconds (str(int(write_date.timestamp() * 1000000))), which If-Match is compared against. A client echoing a
bundle ETag in If-Match gets 409.

This is the same drift as #554 item B (ProgramMembership PUT), fixed in PR #555 for that endpoint only.

Proposal

Use the microsecond format in bundle entries (ideally via the shared version helper proposed in #561), with a
test that a bundle ETag is accepted by the resource's PUT.