feat(contracts): extract admin functions and emit AdminParamsChanged events

[phertyameen]

Summary

Extracts all administrative functions into admin.rs and replaces the
existing per-field events with a single unified AdminParamsChanged event
so the indexer can subscribe to one topic and be notified of any admin
parameter mutation.

Changes

src/admin.rs (new file)

• set_admin - transfer admin privileges
• set_outcome_manager - replace outcome manager
• set_fee - set protocol fee in basis points (new), guards against
  values > 10,000 (100%)

src/events.rs

• Replaces emit_admin_changed / emit_outcome_manager_changed with
  emit_admin_params_changed_address and emit_admin_params_changed_u32
• Both emit on topic ("call_registry", "admin_params_changed")
• Exports PARAM_ADMIN, PARAM_OUTCOME_MANAGER, PARAM_FEE_BPS
  constants for the indexer to match against without magic strings

src/types.rs

• Adds fee_bps: u32 to ContractConfig (default 0 in initialize)

src/lib.rs

• Adds mod admin
• Delegates set_admin / set_outcome_manager to admin.rs
• Exposes new set_fee entry-point
• Seeds fee_bps: 0 in initialize

Event schema

Field	Type	Description
param	Symbol	Which field changed: admin, outcome_manager, fee_bps
changed_by	Address	Admin address that authorised the change
old_value	Address|u32	Previous value
new_value	Address|u32	New value

Testing

• set_admin - config updated, event payload correct
• set_outcome_manager - config updated, event payload correct
• set_fee - config updated, event payload correct
• set_fee(0) - valid
• set_fee(10_000) - valid boundary
• set_fee(10_001) - panics with expected message
• All existing tests still pass (cargo test)

Closes #120

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[PeterOche]

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