feat(gateway,core): recurring/subscription payment flows

[gabvega08]

Summary

Adds a recurring/subscription payment primitive to Connect, so merchants with subscription business models can charge on a schedule through Pacto. Closes #41.

A subscription is a DB-backed plan (amount, interval, asset, FX pair) tied to a merchant + payer. A polling scheduler creates a new escrow on each billing cycle, reusing the quote engine (#18) for FX pricing at charge time, and the subscription lifecycle is surfaced through the existing signed-webhook pipeline. Test mode only for now (live returns 501 not_implemented, consistent with escrows).

What's included

• Data model — Subscription + SubscriptionCharge Prisma models, enums, and a migration.
• Charge cycle (subscriptions/charge.ts) — re-prices via createQuote(), creates an escrow via the test-mode simulator, records the charge, emits events, with bounded retry (SUBSCRIPTION_MAX_ATTEMPTS, default 3) → past_due + stop.
• Scheduler (subscriptions/runner.ts) — polling runner mirroring the webhook delivery runner; wired into index.ts.
• HTTP routes /v1/subscriptions — create / retrieve / list / cancel (session-scoped auth like escrows; idempotency on create).
• Webhook events — subscription.created, subscription.charged, subscription.failed, subscription.canceled through the existing dispatchEvent() pipeline.
• Test controls — /v1/test/subscriptions/:id/advance (deterministic synchronous charge) and /fail-next (simulate insufficient funds).
• SDK — subscriptions resource (create/retrieve/list/cancel) + test-mode helpers in @pacto-connect/core.
• Docs — subscription events + env vars documented in WEBHOOKS.md.

Acceptance criteria

• Auto-escrow each cycle without manual intervention — the runner charges due subscriptions on its poll interval (accelerated in test mode via TESTMODE_SUB_INTERVAL_MS); advance provides a deterministic path for tests.
• Failed charge emits subscription.failed and does not retry indefinitely — bounded retry with backoff, then past_due + a single subscription.failed; the runner never re-selects a non-active subscription.
• Cancel stops future charges and emits subscription.canceled — cancel is terminal and the event is emitted exactly once, only on a real active → canceled transition.

Testing

• Gateway: 149 tests pass. Core: 67 tests pass.
• Monorepo type-check clean (8/8 workspaces); biome check clean.
• Charge/runner/route/domain suites cover each acceptance criterion, session-scoping, live-mode 501, idempotency, and the bounded-retry cap.

Notes / scope

• Test-mode only; live on-chain settlement is out of scope.
• Known test-mode limitation: on gateway restart the in-memory simulator escrows are lost; the DB-backed subscription survives and keeps charging.