implemented TTL and storage ledger extension for escrow

[Ogstevyn]

Summary

• Defined ESCROW_TTL_THRESHOLD (518,400 ledgers ≈ 30 days) and ESCROW_TTL_EXTEND_TO (1,555,200 ledgers ≈ 90 days) in a mod constants block with inline rationale comments
• Escrow amount is now stored in persistent() storage (correct type for escrow records under Stellar's state-rent model) with TTL extended on every write
• Instance storage for admin config (admin, pending_admin) has TTL extended on every admin action: init, propose_admin, and accept_admin
• Added get_amount view function to support persistent storage verification in tests
• Added 3 unit tests covering TTL extension on escrow create and both admin actions

Test plan

• test_init_and_get_status — existing test still passes
• test_propose_and_accept_admin — still passes with TTL calls added
• test_accept_admin_rejected_for_non_pending — still panics correctly
• test_admin_cleared_after_transfer — still passes
• test_admin_transfer_emits_event — still passes
• test_init_persists_escrow_amount_with_ttl — amount is readable from persistent storage after init
• test_propose_admin_extends_instance_ttl — propose_admin succeeds with TTL extension, state correct
• test_accept_admin_extends_instance_ttl — accept_admin succeeds with TTL extension, admin updated

Closes #15

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