Implement subscription smart contract oracle integration for price feeds

[Itodo-S]

Description

Integrate on-chain price oracles for subscription pricing in stablecoins or volatile tokens, ensuring fair market-rate conversions at billing time.

Changes

Oracle Price Feed Integration

• Added OracleContract storage key for registering the oracle contract address
• Added TokenSymbol mapping to resolve token addresses to oracle symbols
• Added set_oracle_contract, clear_oracle_contract, get_oracle_contract admin functions
• Added set_token_symbol, remove_token_symbol, get_token_symbol for token-to-symbol mapping

Price Lookup at Billing Execution Time

• Modified charge_subscription to consult oracle pricing via resolve_charge_price helper
• Uses cached oracle reads (get_price_with_cache with 600s TTL) for gas efficiency
• Gracefully falls back to stored plan price when oracle is unavailable

Slippage Protection

• Added PriceBounds struct with min_price_bps and max_price_bps for per-plan bounds
• Added set_price_bounds, clear_price_bounds, get_price_bounds admin functions
• Charge price is capped within bounds to prevent price manipulation at billing boundary

Oracle Health Monitoring

• Added OracleMonitorService backend service with periodic health checks
• Tracks feed staleness, circuit breaker state, and consecutive faults
• Generates alerts for stale prices, open circuits, and missing price data

Fiat Equivalent Display

• Added fiatPrice, fiatCurrency, fiatPriceUpdatedAt, oraclePriceDeviationBps to Subscription type
• Added OraclePriceService frontend service for enriching subscriptions with fiat prices
• Added totalFiatMonthlySpend and fiatCurrency to SubscriptionStats

Files Changed

• contracts/types/src/lib.rs - Added OracleContract, PriceBounds, TokenSymbol storage keys and PriceBounds struct
• contracts/subscription/Cargo.toml - Added subtrackr-oracle dependency
• contracts/subscription/src/lib.rs - Oracle integration in charge_subscription with slippage protection
• backend/services/oracleMonitorService.ts - Oracle health monitoring service
• backend/services/index.ts - Service export
• src/services/oraclePriceService.ts - Frontend fiat price enrichment service
• src/types/subscription.ts - Fiat price display fields

Closes #370

[drips-wave]

@Itodo-S Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits