Cardano Updates

When an off-chain DRep metadata image is an inline base64 data URI
(`data:<mime>;base64,...`), db-sync stored only the base64 payload in
`off_chain_vote_drep_data.image_url`, stripping the `data:<mime>;base64,`
prefix (and the MIME type), so the stored value was neither the source
contentUrl nor a usable data URI.

CIP-119 defines `contentUrl` as the full data URI for inline images, so
store it verbatim. The `Image` JSON decoder now keeps any `data:`-scheme
contentUrl as-is; regular URL images (which carry a sha256) are unchanged.

Adds a regression test asserting a base64 data-URI contentUrl is decoded
without losing its prefix.

Re-processing the off-chain metadata of an already-stored voting anchor
duplicated its child rows (off_chain_vote_drep_data, off_chain_vote_author,
off_chain_vote_reference, off_chain_vote_external_update,
off_chain_vote_gov_action_data). The parent insert used ON CONFLICT DO
UPDATE ... RETURNING id, so a re-fetch returned the existing id, and the
children - which have no uniqueness and were plain-inserted - were written
again.

Make insertion idempotent: insertBulkOffChainVoteDataReturningNew uses ON
CONFLICT DO NOTHING RETURNING (voting_anchor_id, hash, id) so only newly
inserted parents come back, and the orchestration inserts child rows only
for those, matched by natural key. Adds a WithResultBulkColumns result mode
for a custom RETURNING column list. Prevents new duplicates; existing rows
on synced DBs are unaffected.

Adds a regression test (cardano-db test-db) asserting a re-fetch creates no
duplicate child rows.