Feat: Add backend observability for contract event schema drifts

[estyemma]

Closed: #30

What was implemented
New file: contract-event-drift.service.ts
The core drift detector. It maintains:

A rolling 5-minute window (10 × 30s buckets) with per-reason counters: unknown_event_name, schema_version_too_high, incompatible_schema_version, field_mismatch, topic_mismatch, parse_error
Cumulative totals since service start
A ring buffer of 20 recent drift events (field names only, no raw payload values — safe for logging)
detectFieldDrift() compares observed XDR map keys against the schema registry's payloadKeys and returns missing/extra fields
Alert logging at ERROR level when the rolling rejection rate exceeds 10% with ≥20 events in the window
Modified: soroban-event.parser.ts
Every rejection path now calls driftService.recordDrift(...) with structured context:

Unknown event names → unknown_event_name
Topic namespace mismatch for a known event → topic_mismatch
schema_version > MAX_SUPPORTED_SCHEMA_VERSION → schema_version_too_high
Version not in compatibleVersions → incompatible_schema_version
XDR field names extracted (no values) before parsing → field_mismatch if any payloadKeys are missing
Catch block → parse_error
Successful parse → driftService.recordProcessed()
Modified: metrics.service.ts
Five new Prometheus metrics:

contract_event_parser_rejections_total — labeled by reason, contract_id, event_name, schema_version
contract_event_unknown_names_total — labeled by contract_id (separate from parse errors, per acceptance criteria)
contract_event_field_mismatches_total — labeled by contract_id, event_name, schema_version
contract_event_parse_errors_total — labeled by contract_id
contract_event_rejection_rate gauge — rolling rate per contract_id, updated after each rejected event batch
New file: parser-health.dto.ts
Swagger-annotated DTOs for the health endpoint response.

Modified: soroban-indexer.controller.ts
New GET /indexer/parser-health endpoint returning the full ParserHealthSnapshot — window stats, totals, known event names, schema versions, and recent drift events.

Modified: ingestion.module.ts
ContractEventDriftService registered as a provider and exported
SorobanEventParser now uses a factory provider that wires both the onUnknownSchemaVersion callback (feeding Prometheus) and the ContractEventDriftService at bootstrap time
Both StellarIngestionService and SorobanEventIndexerService receive the shared parser and drift service via DI

[MaryammAli]

@estyemma
run
npm run build and npm run test to fix errors

[estyemma]

@estyemma run npm run build and npm run test to fix errors

@MaryammAli pls check