feat: pre-authorized recurring shielded withdrawals (#151)

[ifygreg01-best]

Adds a complete bounded recurring-withdrawal system so users can pre-authorize a series of scheduled payouts from one shielded note without generating a new ZK proof for each occurrence.

Circuit (circuits/recurring/)

• New Noir circuit that proves note ownership and binds a policy tuple (recipient, max_amount, period_secs, max_uses) into an auth_commitment via a domain-separated Poseidon2 chain (RECURRING_DOMAIN = 0x52454341).
• Consumes the source note nullifier on setup; creates a change note for amount - max_amount. All values 64-bit constrained; max_uses 1-255.
• Nargo.toml + Prover.toml added; compiled artifact stub at frontend/src/circuits/recurring.json (replace with real nargo compile output).

Contract (contracts/pool/src/lib.rs)

• New PoolError variants: AuthNotFound, AuthRevoked, AuthExhausted, PeriodNotElapsed, AmountExceedsCap.
• RecurringAuth contracttype struct stored in persistent storage.
• authorize_recurring: verifies proof, consumes nullifier, inserts change note, stores auth record and recipient address.
• withdraw_recurring: executes one occurrence (payout <= cap, period elapsed, uses_remaining > 0, not revoked); no proof required.
• revoke_recurring: owner-signed immediate revocation.
• get_recurring_auth: read-only view.
• 15 new tests covering every error path and the use-count decrement loop.

Frontend

• lib/notes.ts: RecurringAuth interface + localStorage helpers with advisory locking (getRecurringAuths, saveRecurringAuth, markRecurringAuthRevoked, getActiveRecurringAuths).
• lib/poseidon2.ts: computeAuthCommitment() mirrors hash_auth in-circuit.
• lib/prover.ts: proveRecurringAuthorization() with all 8 circuit inputs; lazy-loads recurring.json so missing artifact only breaks the recurring flow, not the rest of the bundle.
• app/recurring/page.tsx: setup UX (note selector, recipient, amount, period presets, use count, summary) + management view with on-chain state polling and one-click revocation.
• app/api/relay-recurring/route.ts: rate-limited server-side trigger for cron/scheduler use; maps PeriodNotElapsed/AuthExhausted/AuthRevoked to HTTP 200 status:skipped so over-eager schedulers don't alarm.
• components/Header.tsx: Recurring added to nav.

Docs

• docs/THREAT_MODEL.md: new Recurring authorizations section covering circuit guarantees, per-occurrence contract enforcement, exposure comparison vs one-shot withdrawals, and residual risks.

All 154 frontend tests pass.

Summary

Why

Area

Testing done

• just test passes (contracts + frontend)
• just test-e2e passes, if this touches deposit/withdraw/compliance flows
• If a circuit changed: recompiled and regenerated the checked-in frontend/**/circuits/*.json artifacts
• pnpm lint / pnpm build pass, if this touches frontend/

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Related issue

Closes #