Process SAC balance entry changes and store SAC balances

[aditya1702]

What

Add SAC (Stellar Asset Contract) balance tracking for contract addresses (C...) by storing balances directly in PostgreSQL and reading them from the database instead of making RPC calls. This includes a dedicated SACBalancesProcessor for extracting SAC balance changes from ledger entries during live ingestion, and a new sac_balances table for persistent storage.

Why

1. Performance - Reading SAC balances from local PostgreSQL is significantly faster than making RPC calls for each contract address balance
2. Complete state tracking - Stores SAC balance fields (balance, is_authorized, is_clawback_enabled, last_modified_ledger) enabling richer balance queries
3. Consistent with architecture - Follows the same pattern established for native XLM balances and trustlines in `
4. Contract address support - Classic G-addresses have SAC balances in their trustlines; contract C-addresses need separate storage since they don't have trustlines

Key Architectural Changes

BEFORE:

flowchart LR
    A[GraphQL Resolver] --> B[RPC getLedgerEntries]
    B --> C[Parse SAC Balance Map]
    C --> D[Return Balance]

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AFTER:

flowchart LR
    subgraph Ingestion
        I1[SACBalancesProcessor] --> I2[(sac_balances)]
    end
    subgraph Query
        A[GraphQL Resolver] --> B{Address Type?}
        B -->|C-address| C[SACBalanceModel.GetByAccount]
        C --> D[(PostgreSQL)]
        B -->|G-address| E[TrustlineBalanceModel]
    end

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Per-File Change Summary

File	Type	Description
internal/db/migrations/2026-01-16.0-account-sac-balances.sql	✨ New	Creates account_sac_balances table with account_address, contract_id, balance, is_authorized, is_clawback_enabled, last_modified_ledger
internal/data/sac_balances.go	✨ New	SACBalanceModel with GetByAccount, BatchUpsert, BatchCopy methods for CRUD operations
internal/data/mocks.go	📝 Modified	Added SACBalanceModelInterface mock for testing
internal/data/models.go	📝 Modified	Added SACBalanceModel to models initialization
internal/indexer/types/types.go	📝 Modified	Added SACBalanceChange struct and SACBalanceOp enum (ADD, UPDATE, REMOVE)
internal/indexer/processors/sac_balances.go	✨ New	SACBalancesProcessor extracts SAC balance changes from contract data ledger entries using sac.ContractBalanceFromContractData SDK function
internal/indexer/indexer_buffer.go	📝 Modified	Added SACBalanceChangeKey composite key, sacBalanceChangesByKey map, PushSACBalanceChange/GetSACBalanceChanges methods with deduplication
internal/indexer/indexer.go	📝 Modified	Wired SACBalancesProcessor into indexer, calls processor in processTransaction
internal/services/token_ingestion.go	📝 Modified	Added processSACBalanceChanges method, updated ProcessTokenChanges interface to accept SAC balance changes, streams SAC balances during checkpointing
internal/services/contract_metadata.go	📝 Modified	SAC contract metadata now extracted from ledger data (no RPC needed), only SEP-41 contracts need RPC metadata fetch
internal/services/ingest.go	📝 Modified	Updated to pass SAC balance changes to ProcessTokenChanges
internal/services/ingest_live.go	📝 Modified	Added SAC balance changes to BatchTokenChanges struct
internal/services/ingest_backfill.go	📝 Modified	Added SAC balance changes handling during backfill
internal/serve/graphql/resolvers/queries.resolvers.go	📝 Modified	For C-addresses: reads SAC balances from DB; for G-addresses: reads trustlines from DB. Removed RPC calls for SAC balances
internal/serve/graphql/resolvers/account_balances_utils.go	📝 Modified	Added buildSACBalanceFromDB helper, removed parseSACBalance (no longer needed), added sacBalances field to accountKeyInfo
internal/serve/graphql/resolvers/balance_reader.go	📝 Modified	Added GetSACBalances method to BalanceReader interface
internal/serve/graphql/resolvers/account_balances_test.go	📝 Modified	Updated tests to reflect new DB-based SAC balance reading
internal/serve/serve.go	📝 Modified	Added SACBalanceModel to resolver dependencies
internal/utils/utils.go	📝 Modified	Removed unused SAC balance parsing utilities

Issue that this PR addresses

Closes #460