Fix chain import/export to handle duplicates, add metadata fields, and improve datetime parsing

- Add chain_id field to Block creation in import_block endpoint
- Remove await from synchronous session.commit in import_block
- Add _serialize_optional_timestamp helper to handle various timestamp formats
- Add _parse_datetime_value helper with proper datetime parsing and error handling
- Add _select_export_blocks to filter duplicate blocks by height during export
- Add _dedupe_import_blocks to filter

apps/blockchain-node/tests/test_sync.py

@@ -51,6 +51,7 @@ def _seed_chain(session_factory, count=5, chain_id="test-chain", proposer="propo
             ts = datetime(2026, 1, 1, 0, 0, h)
             bh = _make_block_hash(chain_id, h, parent_hash, ts)
             block = Block(
+                chain_id=chain_id,
                 height=h, hash=bh, parent_hash=parent_hash,
                 proposer=proposer, timestamp=ts, tx_count=0,
             )