Fix issues 906, 907, 908, 909: Backend and contract fixes

[kokobutter-web]

This PR addresses four outstanding issues across the smart contracts and backend:

Closes #909

• Adds get_events_batch to event_registry contract to query multiple events at once (capped at 50 IDs to prevent instruction limit issues).
• Adds TooManyIds error variant.

Closes #908

• Enforces filtering of flagged events consistently across all events endpoints and internal verification checks that query by event_id. Flagged events are now completely hidden.

Closes #907

• Fixes expiry checking logic in the multi-sig proposal system. Both approve_proposal (in event_registry) and vote_on_proposal (in ticket_payment) now strictly evaluate proposal expiration before allowing any changes or state checks.

Closes #906

• Adds an is_verified column to the organizers database table.
• Extends the Soroban event listener to handle CollateralStakedEvent and CollateralUnstakedEvent, automatically synchronizing the organizer's verified status from the blockchain to the backend database.

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[Yunusabdul38]

LGTM
thanks for your contribution