Add genesis block existence check to PoA proposer start() method

- Block production now requires a genesis block to exist before starting
- Prevents automatic local genesis creation and subsequent chain divergence
- Follower nodes without genesis will not produce blocks even if proposer_id is present

apps/blockchain-node/src/aitbc_chain/consensus/poa.py

@@ -124,6 +124,14 @@ class PoAProposer:
         if not getattr(settings, "enable_block_production", True):
             self._logger.info("Block production disabled, skipping PoA proposer loop")
             return
+        # Check if genesis block exists before starting proposer loop
+        with self._session_factory() as session:
+            genesis = session.exec(
+                select(Block).where(Block.chain_id == self._config.chain_id).where(Block.height == 0).limit(1)
+            ).first()
+        if genesis is None:
+            self._logger.warning("No genesis block found, skipping PoA proposer loop. Block production requires a genesis block.")
+            return
         self._logger.info("Starting PoA proposer loop", extra={"interval": self._config.interval_seconds})
         await self._ensure_genesis_block()