fix(indexer): mark reads stale and surface per-network status when indexer is unconfigured (#238)

[Stanley471]

Closes #238

Summary of Changes

1. Explicit Staleness Marking on Unconfigured/Stopped Networks:

   • In \src/api.ts, the stale read middleware now checks whether the network's indexer is stopped or unconfigured via \isIndexerStopped(net)\ alongside \checkRpcHealth(net).
   • When the indexer loop for a network is not running, read responses serve whatever data is in the database with explicit staleness headers and body attributes:
     • Header: \X-Data-Stale: true, \X-Indexer-Running: false, and \X-As-Of-Ledger: .
     • JSON body: \stale: true, \indexer_running: false, and \�s_of_ledger: .
   • Rationale for Answering with Explicit Staleness Markers: Answering with an explicit staleness marker rather than refusing outright is essential so existing clients, dashboards, and wallet history views can still display previous historical transactions rather than encountering hard 500/503 errors or receiving an empty list that masquerades as zero transactions. The presence of \stale: true\ and \indexer_running: false\ allows callers and dashboards to unmistakably distinguish 'no transfers' from 'indexer stopped/unconfigured'.

2. **Per-Network Indexer State in /status**:

   • Track per-network indexer status in \src/indexer.ts\ (
     unningNetworks, \isIndexerRunning, \isIndexerStopped, \getIndexerUnstartedReason).
   • In \src/api.ts, the /status\ endpoint now reports every enabled network in the
     etworks\ map with
     unning: boolean, \lastIndexedLedger, \latestLedger, \lagLedgers, and
     eason\ (when stopped/unstarted due to missing configuration).

3. Validation & Unconfigured-RPC Path Tests:

   • Added unit test in \src/tests/multiNetworkIndexer.test.ts\ covering the unconfigured-RPC path when \startAllIndexers\ encounters a network missing required RPC config (e.g. \SOROBAN_RPC_URL_MAINNET).
   • Added unit tests in \src/tests/staleReads.test.ts\ verifying that reads for a stopped indexer return explicit staleness markers and that /status\ surfaces per-network running status, lag, and unstarted reasons.

4. Documentation:

   • Updated \README.md\ to document the per-network RPC variables (\SOROBAN_RPC_URL_TESTNET, \SOROBAN_RPC_URL_MAINNET), their resolution order, and behavior when an enabled network indexer is unconfigured.

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