feat: add contract safeguards, fuzzing, ABI artifact, and refund resolution

[Danlee76]

Summary

• add a contract-level withdrawal circuit breaker with fixed-bucket rolling volume tracking, auto-pause events, admin reset/configuration, and contract tests
• add entrypoint-oriented fuzz coverage plus a CI workflow with bounded PR fuzzing and larger scheduled fuzzing
• add contract ABI/spec artifact generation, freshness checking in CI, release attachment, and docs link to the machine-readable spec
• add creator refund resolution APIs for received requests, approval/rejection prepare flows, signed transaction submit flows, OpenAPI paths, and backend tests for authorization, invalid transitions, and pagination

Testing

• git diff --check
• attempted cargo test -p tipz-contract test_circuit_breaker, but local Rust 1.88.0 is partially installed and rustup stalls while repairing/downloading components
• attempted backend npm test -- refunds.test.ts, but backend dependencies were missing and npm install timed out before installing Vitest

Closes #1191
Closes #1196
Closes #1197
Closes #1198

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[Akanimoh12]

Please close your pr well and open pr against the right branch, not main

[Akanimoh12]

please resolve conflicts

[gitguardian]

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