all: Store native XLM balances and trustline data in PostgreSQL

[Copilot]

What

Implements database storage for native XLM balances and refactors token ingestion to eliminate RPC calls for balance queries.

Why

Balance queries currently hit RPC for every account lookup. This creates performance bottlenecks and unnecessary external dependencies for data we're already processing during ingestion.

Architecture changes:

• Native balance tracking: New native_balances table stores XLM balances, minimum reserves, and liabilities extracted from account ledger entries via AccountsProcessor
• Separated token storage: Split monolithic account_tokens into three specialized models:
  • native_balances - XLM account data
  • trustline_balances - Classic trustline holdings with full XDR fields
  • account_contract_tokens - SAC/SEP-41 contract mappings
• Map-based deduplication: Changed IndexerBuffer from slice-based to map-based storage, keeping only the latest change per account/trustline per ledger. Handles CREATE→REMOVE no-ops within same ledger.
• Database-first queries: GraphQL resolvers read from PostgreSQL via BalanceReader interface instead of RPC

Data flow:

Ledger ingestion → AccountsProcessor extracts balance changes → IndexerBuffer deduplicates → 
BatchUpsert to native_balances table → GraphQL queries read from DB

GraphQL schema additions:

type NativeBalance {
    minimumBalance: String!
    buyingLiabilities: String!
    sellingLiabilities: String!
    lastModifiedLedger: Int!
}

Minimum balance calculation:

minimumBalance = (2 + numSubEntries + numSponsoring - numSponsored) * 5_000_000 + sellingLiabilities

Known limitations

N/A

Issue that this PR addresses

N/A

Checklist

PR Structure

• It is not possible to break this PR down into smaller PRs.
• This PR does not mix refactoring changes with feature changes.
• This PR's title starts with name of package that is most changed in the PR, or all if the changes are broad or impact many packages.

Thoroughness

• This PR adds tests for the new functionality or fixes.
• All updated queries have been tested (refer to this check if the data set returned by the updated query is expected to be same as the original one).

Release

• This is not a breaking change.
• This is ready to be tested in development.
• The new functionality is gated with a feature flag if this is not ready for production.

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