fix: MEV mitigation, invoice count safety, batch reputation, pause ch…

[Anthony-19]

Issue #MEV-1 — Mitigate MEV/front-running in resolve_fund_queue

• Add QUEUE_DELAY_LEDGERS = 120 constant (~10 min at 5s/ledger) to constants.rs
• Add QueueNotMature = 39 error variant to errors.rs
• Add DataKey::FundQueueOpenedAt(u64) storage key to storage.rs
• Add try_set_fund_queue_opened_at / get_fund_queue_opened_at helpers in
  both storage.rs and invoice.rs
• join_fund_queue records the ledger sequence when the first LP joins
• resolve_fund_queue enforces the maturity delay; rejects with QueueNotMature
  if the delay has not elapsed, preventing same-block front-running
• Add FundQueueResolutionAttempted event (emitted on both success and failure)
  so off-chain monitors can detect MEV probing
• Tests: tests_mev_mitigation.rs (10 tests covering: reject before delay,
  succeed after delay, timer anchored to first join, idempotency, events)

Issue #invoice-count — Fix get_invoice_count underflow before initialization

• get_invoice_count now uses saturating_sub(1) so an uninitialized contract
  (read_next_invoice_id defaults to 1) returns 0 instead of panicking
• Tests: tests_invoice_count.rs (5 tests covering: uninitialized, post-init,
  after first submit, N submits, batch submit count)

Issue #batch-reputation — Fix batch_submit not incrementing invoices_submitted

• Add increment_invoices_submitted(&env, &params.freelancer) inside the
  submit_invoices_batch loop, mirroring the single submit_invoice path
• Tests: tests_batch_submit_reputation.rs (5 tests covering: 3-invoice batch,
  cumulative batches, two freelancers in one batch, parity with single submit,
  no paid/defaulted side-effects)

Issue #pause-checks — Add missing pause guards to expire_invoice and appeal_default

• expire_invoice: add is_paused check at function entry, returns ContractPaused

• appeal_default: add is_paused check at function entry, returns ContractPaused

• Tests: tests_pause_checks.rs (8 tests covering: pause blocks expire,
  expire succeeds when unpaused, expire after unpause, pause blocks appeal,
  appeal succeeds when unpaused, appeal after unpause, both blocked simultaneously)

  Closes Add missing pause checks in expire_invoice and appeal_default #605
  Closes Fix batch_submit not incrementing invoices_submitted for submitters #606
  Closes Fix get_invoice_count underflow panic before contract initialization #607
  Closes Mitigate MEV/front-running in resolve_fund_queue #608

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