feat: add initialize, admin, and payment_token entry points

[phertyameen]

Closes #599

Summary

Implements the one-time initialize entry point and two public query
functions on top of the Issue #598 scaffold.

Changes

• initialize(env, admin, payment_token) — guards against double-init,
  requires admin auth, writes both addresses to instance storage, and
  emits an init event
• admin(env) — delegates to Self::get_admin(&env)
• payment_token(env) — delegates to Self::get_payment_token(&env)

Acceptance Criteria

• cargo check -p workspace_booking passes
• cargo clippy -p workspace_booking -- -D warnings passes
• Double initialize panics with Error(Contract, #3)
• admin() returns the address passed to initialize
• payment_token() returns the token address passed to initialize
• init event is emitted on successful initialization

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