feat(contracts): fix #160 payment scheduler and #177 price oracle

[Dopezapha]

Closes #160
Closes #177

PR Description

Fixes #160 by implementing an escrow-backed payment scheduler for one-time and recurring payments. Payers fund schedules up front, anyone can execute due schedules, and pending schedules can be cancelled or modified with proper authorization and escrow reconciliation.

Fixes #177 by implementing a Soroban price oracle aggregator for payment routing. The oracle supports trusted manual sources and external SEP-40-style oracle contracts, aggregates valid prices with median filtering, rejects stale/outlier data, and falls back to the last good aggregate when live sources are unavailable.

Changed

• Contract: Add Payment Scheduling Contract #160 Added escrow-backed one-time and recurring payment schedules.
• Contract: Add Payment Scheduling Contract #160 Added cancellation, modification, pause/admin controls, validation, checked arithmetic, TTL handling, and focused tests.
• Contract: Implement Oracle Price Feed Integration #177 Added manual and SEP-40-style oracle source support.
• Contract: Implement Oracle Price Feed Integration #177 Added price normalization, median aggregation, staleness checks, deviation filtering, last-good fallback, source management, and focused tests.
• Added the correctly cased contracts/payment-history/Cargo.toml required for the contracts workspace to load during tests.

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[ONEONUORA]

Great job @Dopezapha