feat: implement per-market cumulative extension cap (#672)

[Awointa]

Summary

closes #672
Adds a configurable cumulative extension cap per market, preventing a single
market from being extended beyond an admin-set total day limit across all extension calls.

Changes

storage.rs

• Added DataKey::MarketExtensionTotal(Symbol) — persistent counter tracking total days
  extended per market.

extensions.rs

• Added ExtensionValidator::check_cumulative_cap — reads the admin-configured cap
  (cum_ext_cap) from persistent storage; a value of 0 disables the cap. Emits an audit event
  before rejecting a cap-exceeding request.
• Added ExtensionUtils::increment_extension_total — increments the per-market counter after
  each successful extension.
• Wired both into ExtensionManager::extend_market_deadline, after the existing per-call
  limit check.

err.rs

• Added CumulativeExtensionCapHit = 506 with message and code string.

extensions_cumulative_cap_tests.rs

• Full test coverage: no cap set (unrestricted), cap enforced on overflow, running total
  correctness, audit event emitted on rejection.

Testing

cargo test -p predictify-hybrid

All existing and new tests pass.

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[greatest0fallt1me]

Merged via direct push to master (admin)