Widen blocks.header so larger block headers can be indexed

[kwsantiago]

blocks.header is varchar(160), which fits exactly an 80 byte header. A chain whose headers are larger cannot be indexed: every write fails with Data too long for column 'header', and $updateBlocks retries the same block indefinitely.

Widens the column to 512 for both fresh installs and existing databases (schema version 112). Gated on isBitcoin like the neighboring migrations. varchar stores only the bytes used, so mainnet is unaffected.

Verified against MariaDB 10.5.21:

	schema_version	blocks.header
before	111	varchar(160)
after, same db	112	varchar(512)
after, empty db	112	varchar(512)

npm run test:with-db passes.

[Retropex]

Thanks but already implemented here 7976b28#diff-0d1d81dbd6a8a15fc2c0bf4598b3c0583c9867ce082386be775af0f978a7dfb3