feat(opencode): move to the batched GetBalances RPC

[bmc08gt]

Blocked on code-payments/ocp-client-protocol#6. Draft until 0.4.0 is published, because this pins a version that does not exist yet — Gradle resolution reports com.flipcash:ocp-client-protocol:0.4.0 FAILED until then. CI never sees the local override, since protoLocalRoot lives in untracked local.properties.

Why

ocp-protobuf-api removed GetBalance in favour of GetBalances (#66). The Balance service had exactly one RPC, so there is no way to take the new package without moving the call sites.

The request goes from a single owner to repeated owners (1-1024) plus an optional mints filter. The response goes from a flat core_mint_value to map<string, OwnerBalance> balances_by_owner, where each entry keeps a core_mint_value total and adds a balances_by_mint breakdown.

What changed

All under services/opencode/src/main/kotlin/com/getcode/opencode/:

File	Change
internal/network/api/BalanceApi.kt	getBalance(owner) → getBalances(owners, mints = emptyList()), built with addAllOwners/addAllMints
internal/network/services/BalanceService.kt	maps the new Result enum, returns Result<List<OwnerBalance>> from balancesByOwnerMap.values
model/financial/OwnerBalance.kt	new domain type mirroring the proto's two-level shape
internal/domain/mapping/OwnerBalanceMapper.kt	new mapper, reusing the existing SolanaAccountId.toPublicKey()/.toMint() extensions
repositories/BalanceRepository.kt, internal/domain/repositories/InternalBalanceRepository.kt	plural signature
controllers/BalanceController.kt	plural signature; still takes bare PublicKeys, as the RPC stays unauthenticated
model/core/errors/Errors.kt	GetBalanceError → GetBalancesError per the repo's Get<Rpc>Error convention, NotFound case removed

NOT_FOUND is gone from the proto Result enum. OK and DENIED keep 0 and 1, so nothing renumbered, but an owner with no balances is now absent from the map rather than signalled by a result code. InternalBalanceRepository now only special-cases Denied before reporting.