feat: process pending signatures queue and accept automatic deposits

[lpahlavi]

Summary

• Implements process_pending_signatures timer function: iterates the PENDING_SIGNATURES queue in round-robin order, pops one signature per account per round, validates each via fetch_and_validate_deposit, emits AcceptedDeposit { source: Automatic } on success, silently discards invalid signatures, and reschedules itself at Duration::ZERO if more remain
• Adds TaskType::ProcessPendingSignatures to the task set
• Skips signatures whose deposit_status is already Some(_) to avoid double-processing
• Wires a new 5-second PROCESS_PENDING_SIGNATURES_DELAY timer in main.rs
• 4 unit tests: valid deposit accepted, invalid discarded, already-processed skipped, reschedule when more remain
• Integration test verifying the full update_balance → poll → signature processing → AcceptedDeposit event flow

Stack

This PR stacks on top of:

1. refactor: add source field to AcceptedDeposit event #179 — add source field to AcceptedDeposit event (rebased on)
2. feat: add pending-signatures queue for automated deposit flow #175 — pending-signatures queue (cherry-pick; drop when feat: add pending-signatures queue for automated deposit flow #175 merges)
3. refactor: extract shared deposit validation from manual flow #174 — extract shared deposit validation (cherry-pick; drop when refactor: extract shared deposit validation from manual flow #174 merges)

Test plan

• cargo test -p cksol_minter --lib — all tests pass
• CI green

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