refactor(x402): split 2720-LOC god-file into class-based middleware (P3B.1)

Move app/routers/x402_enforcement.py verbatim to app/billing/x402/enforcement.py
with a thin X402Enforcer class wrapper exposing the public API.

The legacy app/routers/x402_enforcement.py becomes a re-export shim.
Public API preserved (TOOL_PRICES, CHAIN_USDC, check_trial, verify_payment,
x402_enforcement_middleware, etc.). Routes unchanged (56).

Phase 3B of AUDIT-2026-Q3.md.

app/billing/__init__.py

@@ -0,0 +1,4 @@
+"""Billing subsystem.
+
+Phase 3B of AUDIT-2026-Q3.md.
+"""

app/billing/x402/__init__.py

@@ -0,0 +1,31 @@
+"""x402 billing subsystem.
+
+Phase 3B of AUDIT-2026-Q3.md.
+
+Re-exports the canonical public API of the x402 payment enforcement
+middleware. Implementation lives in app.billing.x402.enforcement
+(moved verbatim from app.routers.x402_enforcement on 2026-07-07).
+"""
+from app.billing.x402.enforcement import (  # noqa: F401
+    CHAIN_USDC,
+    EVM_PAY_TO,
+    SECURITY_HEADERS,
+    SOL_PAY_TO,
+    TOOL_PRICES,
+    VERIFIER,
+    X402Enforcer,
+    build_402_response,
+    check_idempotency,
+    check_trial,
+    consume_trial,
+    get_redis_async,
+    get_revenue,
+    get_trial_status,
+    parse_x_pay_header,
+    request_refund,
+    self_verify_evm_usdc,
+    verify_payment,
+    verify_payment_via_router,
+    x402_discovery,
+    x402_enforcement_middleware,
+)