Cardano Updates

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

Adds an optional `base_path` field to `MinibfConfig` that nests all
minibf routes (including `/health` and `/metrics`) under a configurable
path prefix. Set to `/api/v0` for full Blockfrost OpenAPI compliance;
when omitted, routes remain at the root (no behavioral change).

Invalid values (empty, just `/`, missing leading `/`, or containing `*`)
return a new `ServeError::ConfigError` variant instead of panicking.

Adds an optional `base_path` field to `MinibfConfig` that nests all
minibf routes (including `/health` and `/metrics`) under a configurable
path prefix. Set to `/api/v0` for full Blockfrost OpenAPI compliance;
when omitted, routes remain at the root (no behavioral change).

Invalid values (empty, just `/`, missing leading `/`, or containing `*`)
return a new `ServeError::ConfigError` variant instead of panicking.

Addresses review feedback on the per-UTxO block_ref population added in the
previous commit:

- Introduce `AsyncQueryFacade::block_meta_by_tx_hash` returning chain-point
  metadata (`BlockRefMeta`) decoded once inside the blocking task. Eliminates
  the redundant `MultiEraBlock::decode` previously performed in the gRPC
  caller. (review A-01)

- Hoist `load_era_summary` to the request handler scope via a new shared
  `serve::grpc::block_refs` module. Previously `get_slot_timestamp` reloaded
  the era summary from state on every UTxO; now it runs once per request.
  (review A-02)

- Deduplicate block lookups by source tx hash. A `search_utxos` page whose
  UTxOs cluster on M unique txs now does M lookups instead of N. (review A-03)

- Propagate storage failures as `Status::internal` instead of swallowing
  them. Block-decode failures (indicating archive corruption) degrade to
  `debug!` rather than per-UTxO `warn!` log spam. (review A-04, A-05)

- Extract the entire flow into `serve::grpc::block_refs::fetch_block_refs`,
  shared by both v1alpha and v1beta. Each version maps the chain-agnostic
  `BlockRefData` to its own proto-specific `ChainPoint` via a tiny
  `to_chain_point` helper. (review A-06)

`into_u5c_utxo` now takes the pre-resolved `Option<ChainPoint>` as a
parameter rather than performing its own lookup, keeping the per-UTxO path
cheap.

Tests deferred: no harness exists for `read_utxos`/`search_utxos`. The
`minibf::TestDomainBuilder` pattern can seed a `ToyDomain` via
`import_blocks`, and `FaultyToyDomain` covers error branches — happy to add
unit tests if maintainers prefer them in-PR.