feat(keeper): add retry logic with exponential backoff for failed charges

[VedantMadane]

Summary

Add retry logic to the keeper so transient failures do not silently drop a scheduled charge.

Changes

• keeper/retry.js (new) — exponential backoff helper with permanent vs retryable classification
• keeper/keeper.js — charge path uses withRetry (max 3 attempts); before retrying, confirms via getTransaction that a prior submission did not already succeed (no double-charge)
• keeper/simulate_retry.js — local simulation of transient recovery, permanent skip, and already-SUCCESS guard

Behaviour

Failure	Action
Network/RPC timeout, 5xx, NOT_CONFIRMED	Retry with backoff (cap 3)
Unauthorized, insufficient allowance, cancelled, not due	Log and skip (no retry)
Prior tx hash already SUCCESS	Do not retry

Simulated run log

=== simulate transient RPC failure then success ===
charge(sub=0): attempt 1/3 failed: network timeout talking to RPC — will retry
charge(sub=0): backing off 10ms before attempt 2
charge(sub=0): attempt 2/3 failed: network timeout talking to RPC — will retry
charge(sub=0): backing off 21ms before attempt 3
charge(sub=0): succeeded on attempt 3/3
result: { status: 'SUCCESS', hash: 'SIMULATED_HASH' }

=== simulate permanent insufficient allowance ===
permanent (attempt 1): HostError: Error(Contract, #2) insufficient allowance
charge(sub=1): attempt 1/3 failed: ... — giving up

=== simulate prior tx already SUCCESS (no double-charge) ===
prior tx ALREADY_LANDED already SUCCESS — not retrying
gave up after sends= 1

Run: node keeper/simulate_retry.js

Fixes #7