docs: PRD-033 BIAN-aligned gateway architecture (#1341)

* docs: PRD-033 BIAN-aligned gateway architecture

Define clear boundaries between three gateway concerns:
- api-gateway (inbound HTTP/gRPC proxy, renamed from gateway)
- financial-gateway (new service for Stripe, SWIFT, ACH)
- operational-gateway (non-financial dispatch: meters, IoT, SFTP)

Includes migration plan, Stripe code disposition, shared dispatch
library extraction, and BIAN service domain mapping.

* fix: address CodeRabbit review comments on PRD-033

- Disambiguate Phase 3 label: reconciliation extraction relabelled
  as "future phase" since Phase 3 is the gateway rename
- Fix critical-path estimate: 5+8+2=15 points, not ~13

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Co-authored-by: Ben Coombs <bjcoombs@users.noreply.github.com>

Author: bjcoombs