feat: BaseMail v2 — Attention Bond mechanism

Smart contract:
- AttentionBondEscrow.sol: USDC escrow, 7-day response window,
  whitelist, dynamic pricing, protocol fee (10%)

Database:
- attention_config: per-account pricing params (p₀, α, β, γ)
- attention_bonds: escrow tracking (active/refunded/forfeited)
- attention_whitelist: sender exemptions
- sender_reputation: per-pair reply rates
- qaf_scores: Quadratic Attention Value cache

API (11 new endpoints):
- GET /api/attention/price/:handle — dynamic attention pricing
- GET /api/attention/price/:handle/for/:sender — sender-specific price
- GET /api/attention/qaf/:handle — QAF score (breadth premium)
- PUT /api/attention/config — configure attention bonds
- POST /api/attention/bond — record bond deposit
- POST /api/attention/reply/:email_id — mark reply → refund
- GET/POST/DELETE /api/attention/whitelist — manage exemptions
- GET /api/attention/stats — bond dashboard

Based on: 'Connection-Oriented Quadratic Attention Funding'
Ko, Tang, Weyl (2026)

Author: CloudLobster

ATTENTION-BOND-PLAN.md

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+# BaseMail v2: Attention Bond Implementation Plan
+
+## Overview
+Add the Attention Bond layer to BaseMail: "from agents have email → email has a price."
+
+## Architecture
+
+### 1. Smart Contract: `AttentionBondEscrow.sol`
+- USDC escrow for attention bonds (Base Mainnet)
+- Sender deposits bond → 7-day window → recipient replies (refund) or ignores (forfeit)
+- Whitelist: exempted senders (zero bond)
+- Protocol fee τ = 10%
+
+### 2. DB Schema Changes
+- `attention_config` table: per-account attention price settings
+- `attention_bonds` table: bond escrow tracking
+- `whitelist` table: sender whitelist per account
+- `reputation` table: sender reputation scores (reply rates)
+- `qaf_scores` table: per-account QAF attention value cache
+
+### 3. API Changes
+
+#### New Endpoints
+- `GET /api/attention/:handle` — Get attention price for a recipient
+- `PUT /api/attention/config` — Set your attention price (p₀, α, β, γ)
+- `GET /api/attention/qaf/:handle` — Get QAF score for a recipient
+- `POST /api/attention/bond` — Submit bond tx_hash when sending
+- `POST /api/attention/reply/:email_id` — Mark reply → trigger refund
+- `GET /api/attention/whitelist` — List whitelist
+- `POST /api/attention/whitelist` — Add to whitelist
+- `DELETE /api/attention/whitelist/:address` — Remove from whitelist
+- `GET /api/attention/stats` — Dashboard: bonds received/refunded/forfeited, QAF score
+
+#### Modified Endpoints
+- `POST /api/send` — Add optional `attention_bond: { tx_hash, amount }` field
+- `GET /api/inbox` — Include bond status per email (bonded/refunded/forfeited)
+
+### 4. Smart Contract Design
+
+```solidity
+AttentionBondEscrow {
+  // Core
+  deposit(recipient, emailId, amount) → escrow USDC
+  reply(emailId) → refund (1-τ) to sender, τ to protocol
+  forfeit(emailId) → after 7 days, transfer to recipient
+  
+  // Config
+  setAttentionPrice(basePrice) → set p₀
+  setWhitelist(sender, status) → exempt/unexempt
+  
+  // View
+  getBond(emailId) → bond details
+  getAttentionPrice(recipient) → current price
+}
+```
+
+### 5. QAF Calculation (off-chain, worker)
+- On each bond deposit, recalculate recipient's QAF: AV = (Σ√bᵢ)²
+- Store in `qaf_scores` table
+- CO-QAF: when sender graph data available, apply α_ij discount
+
+### 6. Dynamic Pricing (worker)
+- p(t,s) = p₀ · (1 + α·D(t))^β · (1 - γ·R̄ₛ(t))
+- D(t): message count in rolling 7-day window
+- R̄ₛ(t): sender's historical reply rate with this recipient
+
+## Implementation Order
+1. ✅ Smart contract (`AttentionBondEscrow.sol`)
+2. ✅ DB schema migration
+3. ✅ Attention config & whitelist APIs
+4. ✅ Bond deposit verification in send flow
+5. ✅ Reply/forfeit flow
+6. ✅ QAF calculation
+7. ✅ Dynamic pricing
+8. ✅ Stats dashboard API