Fix: add missing ContractInitialized and PauseToggled events across all contracts

[Songu3020]

#closes
#29

Problem

Several state-changing operations across the six contracts didn't emit events, creating an asymmetry that broke off-chain indexers relying on events as the source of truth:

• reward-pool::set_pause — toggled global pause with no event.
• reward-pool::initialize — bootstrap with no event.
• course-registry::initialize — same.
• badge-nft::initialize — same.
• stake-vault::initialize, quest-engine::initialize, governance::initialize — these already emitted init events correctly.

With half the contracts silent on init and pause, indexers had no way to reconstruct admin wiring from events alone and were forced to poll instead of reacting to the moment a contract was initialized or paused.

Fix

Added two new, consistently-named event types emitted across all six contracts:

• ContractInitialized { admin, role_addresses... } — now emitted by every initialize(), including the three that previously emitted nothing (reward-pool, course-registry, badge-nft), and left unchanged in behavior for the three that already emitted an init event.
• PauseToggled { admin, status } — now emitted symmetrically by every set_pause(), including reward-pool::set_pause (previously silent) and quest-engine::set_pause (brought in line with the others).

Naming follows the ContractInitialized, PauseToggled, AdminTransferred convention noted in the issue, keeping this consistent with the related two-step admin transfer work (#20).

Files changed

• All six contracts: reward-pool, course-registry, badge-nft, stake-vault, quest-engine, governance
• Cross-contract indexer documentation — updated to document the new/standardized events

Testing

• Added/updated tests asserting ContractInitialized is emitted on every contract's initialize().
• Added/updated tests asserting PauseToggled is emitted on every contract's set_pause(), including reward-pool and quest-engine.
• Existing tests for stake-vault, quest-engine, and governance init events remain green — no behavior change for contracts that already emitted correctly.