[Contract] Dispute escalation tiers + arbiter staking/slashing

[Gbangbolaoluwagbemiga]

Goal

Make dispute resolution economically honest and scalable: add escalation tiers and put arbiters'
own funds at stake so good-faith, timely rulings are rewarded and bad/absent ones are penalized.

What needs to be done

• Staking: arbiters must lock a stake to be eligible (extends authorize_arbiter in admin.rs)
• Slashing: an arbiter who is overturned on escalation, or who fails to vote within a deadline,
  loses part of their stake; honest/timely arbiters earn a fee share
• Escalation tiers: Tier 1 = assigned arbiters (voting [Contract] Implement arbiter voting mechanism for dispute resolution #3); Tier 2 = larger panel / owner
  review on appeal within a window
• Track per-arbiter stats (rulings, overturn rate) for reputation

Acceptance criteria

• Eligibility requires an active stake
• Overturned/absent arbiters are slashed; correct ones rewarded — all unit-tested
• Escalation moves a dispute to the next tier within the configured window

Relevant files

contracts/secureflow/src/admin.rs, refund_system.rs, ties to #3 (arbiter voting)