feat(contract): CT-14 VerificationWebhookNotifier and CT-15 CacheWarmupService

[Ibinola]

Summary

Implements [CT-14] and [CT-15] for the Stellar Wave program.

CT-14 — VerificationWebhookNotifier (contract/src/module/webhook/)

• Reads WEBHOOK_URL env var on startup; disabled with a WARN log if not set
• notify(event_type, document_hash, tx_hash, timestamp) fires a POST with JSON payload { event_type, document_hash, tx_hash, timestamp, service: "smalda-contract" }
• Uses reqwest with a 5-second timeout
• Retries once after a 2-second delay on first failure; logs WARN but never propagates errors to the handler
• Called in submit_document, revoke_document, and record_transfer after successful Stellar transactions

CT-15 — CacheWarmupService (contract/src/module/cache_warmup/)

• Runs at application startup before the HTTP server begins accepting connections
• Scans Redis for all submit:* keys, limits warmup to 200 entries
• Writes verified=true cache entries using the stored tx_hash without re-querying Stellar
• Logs the number of warmed entries and total time at INFO level
• Gracefully skips if Redis is unavailable (WARN log, does not block startup)

Testing

• cargo build — clean
• cargo test — 28/28 passing

Closes #546
Closes #547

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