feat(minibf): add /governance/proposals endpoints

[slowbackspace]

Closes #1093
Closes #1098

What

Adds the two Blockfrost governance proposal lookups to MiniBF:

• GET /governance/proposals/{tx_hash}/{cert_index}
• GET /governance/proposals/{gov_action_id} — CIP-0129 bech32 id (gov_action1...), both the 33-byte form and the minimal 32-byte form (index 0, used by cexplorer)

Both share one lookup path: parse the id, read ProposalState for the lifecycle fields, and decode the original proposal tx from the archive for the action detail.

Mapping notes

Blockfrost copies these columns from db-sync, so db-sync source and the official test fixtures are the reference:

• expiration = ledger expiresAfter + 1, which equals dolos ProposalState::expires_at().
• db-sync stamps ratified_epoch and enacted_epoch with the same enactment-boundary epoch. Dolos stores the epoch before that boundary, so both fields map to ratified_epoch + 1 and stay null until the boundary passes.
• dropped_epoch = canceled_epoch (competing action enacted), or expired_epoch + 1 — db-sync drops an expired proposal one epoch after it marks it expired (verified against the official fixtures).
• governance_description is the ledger Aeson encoding of the submitted GovAction ({"tag": ..., "contents": ...}). The endpoint decodes the archived proposal tx to reproduce it exactly: guardrails script hash, lineage parent, and the conway pparams-update JSON names (stakePoolTargetNum, ...) per the cardano-ledger pparams-update.json golden. Ratios render as plain numbers when the fraction terminates, else as numerator/denominator objects, matching the ledger encoding.
• When the archived tx is unavailable (pruned archive, pre-Conway update rows), governance_description is null and the type falls back to ProposalState.

Testing

• Standard matrix per handler: happy path, 400, 404, 500, with a seeded ProposalState; plus unit tests for the description JSON, the ratio rendering, and the minimal CIP-0129 encoding.
• cargo +nightly fmt, clippy --all-targets -D warnings, full workspace tests pass.
• Official blockfrost-tests suite (preview): all 19 governance proposal fixture requests pass — every action type, both id forms per proposal, and both CIP-0129 encodings. Verified against a fully synced preview node running this change. Note: the current fixtures contain no ratified/enacted proposal, so the ratified/enacted mapping is grounded in db-sync source only.

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