Feature/platform enhancements

[Mosas2000]

Implement real-time blockchain event indexing, liquidity pool analytics, GDPR-compliant data export, and dynamic fee configuration

Closes #180
Closes #179
Closes #178
Closes #177

Overview

This PR implements four backend features across the feature/platform-enhancements branch. Each feature is self-contained with its own models, SQL migration, service layer, HTTP handlers, and unit tests.

#180 — Real-time blockchain event indexing

Files: models/indexed_event.go, migrations/sql/0027_create_event_index.up.sql, services/event_indexer.go

Adds IndexedEvent and EventCheckpoint GORM models to persist indexed on-chain events and track the last processed ledger per contract
SQL migration creates indexed_events and event_checkpoints tables with appropriate indexes
EventIndexer service polls Horizon/Soroban RPC on a configurable ticker interval with a channel-based worker pool (3 workers) for concurrent event processing
Handles chain reorganisations by comparing stored ledger sequences against the checkpoint; missed events are recovered on the next poll cycle
Retry logic with exponential backoff on transient RPC failures
Indexer is started as a background goroutine in router.go at server boot

#179 — Comprehensive analytics for liquidity pools

Files: models/pool_metrics.go, services/pool_analytics.go, handlers/liquidity.go, handlers/liquidity_test.go

Adds PoolMetrics and PoolComparison value types for computed analytics
PoolAnalyticsService calculates TVL, 24h/7d/30d volume, fees, APY, and impermanent loss ratio from swap history
GET /api/v1/liquidity/pools/:id/analytics returns full metrics for a single pool
GET /api/v1/liquidity/pools/compare?pool_a=:id&pool_b=:id returns a side-by-side comparison with APY and TVL deltas
Unit tests verify APY and volume calculations and the comparison response shape

#178 — GDPR-compliant data export for users

Files: models/export_job.go, services/data_export_service.go, handlers/gdpr.go, handlers/gdpr_test.go

Adds ExportJob model tracking async export state (pending → processing → completed / failed)
DataExportService gathers all user personal data (profile, KYC record, balances, listings, transactions), serialises to JSON or CSV-in-ZIP, and encrypts at rest using AES-256-GCM
A unique download token (32 random bytes, hex-encoded) is generated per job with a 7-day expiry
POST /api/v1/legal/gdpr/export initiates the job asynchronously; the user receives a download link by email on completion
GET /api/v1/legal/gdpr/export/:id returns current job status
GET /api/v1/legal/gdpr/export/download/:token decrypts and streams the archive; expired or invalid tokens are rejected with 400
Unit tests cover JSON export, CSV/ZIP export, and expired-link rejection

#177 — Dynamic fee configuration with admin interface

Files: models/fee_config.go, migrations/sql/0013_create_fee_config.up.sql, services/fee_service.go, handlers/fee_admin.go, handlers/fee_admin_test.go

Adds FeeConfig model supporting four fee types (marketplace, transfer, staking, liquidity) with BaseBasisPoints, MinBasisPoints, MaxBasisPoints, and a JSON-stored VolumeTier slice for graduated discounts
Adds FeeAuditLog model; every create, update, and deactivation writes a full JSON diff with the acting admin ID and an optional reason
FeeService exposes CRUD, CalculateFee (selects the highest matching volume tier and clamps to min/max), GetActiveFeeConfig, and paginated audit log retrieval
Admin endpoints under POST|GET|PUT|DELETE /api/v1/admin/fees (requires role=admin)
Public endpoints GET /api/v1/fees/preview and GET /api/v1/fees/active allow clients to display the effective fee before submitting a transaction
Configs are soft-deactivated rather than deleted to preserve the audit trail
Unit tests cover create/list, update with audit diff, deactivation, fee preview with and without tier discount, and validation error cases

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[parkerwinner]

fix conflicts @Mosas2000