indexer: count and log decoder fallbacks to unknown/raw_topics

[samuel2926i39-art]

Summary

decode() in indexer/src/decoder.js silently falls back to function: "unknown" (with only raw_topics) whenever it can't recognize an event's function. There was no signal — counter or log — when this happens, so decoding coverage regressions (an unregistered ABI, an unrecognized event signature, a decoder bug) could go unnoticed indefinitely.

This adds:

• soroban_decode_fallback_total{contract_id="..."} — a Prometheus counter (in indexer/src/metrics.js), incremented via a new recordDecodeFallback() helper every time decode() hits the fnName === "unknown" path.
• A structured debug log per fallback (component: "decoder", event: "decode_fallback", contract_id, ledger, tx_hash) for sampling/investigation.
• A ## Decode Fallback Metric section in README.md documenting what a rising rate indicates.

The counter registers into the indexer's existing shared prom-client Registry, so it's automatically exposed at the indexer's existing GET /metrics (indexer/src/api.js) — no additional wiring needed there.

Context / before-after

• Before: an unrecognized event decoded with no observable signal — you'd only notice via a support ticket or a manual DB query for function = 'unknown'.
• After: soroban_decode_fallback_total shows up per-contract in the indexer's metrics scrape, and each occurrence is logged with enough context (contract, ledger, tx) to investigate immediately.
• No behavior change to what gets stored or returned for any event — this is purely additive observability.

Testing

• Added indexer/tests/decodeFallback.test.js (Node's built-in test runner, matching this repo's existing decoder test conventions):
  • isDecodeFallback("unknown") is true; isDecodeFallback("transfer") is false.
  • recordDecodeFallback() increments soroban_decode_fallback_total labeled by contract_id.
  • Counters for different contracts stay isolated (per-contract labeling).
  • A known event never reaches recordDecodeFallback (gate mirrors the real decode() check).
  • recordDecodeFallback() emits the expected structured debug log shape.
• Ran locally: node --test tests/decodeFallback.test.js → 6/6 passing.
• Regression-checked the existing decoder suites for this change: node --test tests/decoder.test.js tests/decoder-parity.test.js tests/scval.test.js (102/102 passing) and node --test test/decoder.sep41.test.js test/sac.test.js (23/23 passing).
• npm run typecheck (indexer) — clean.

Closes #51