docs(architecture): add top-level technical design doc
Closes round 4 of doc-prep. ARCHITECTURE.md is the day-1 read for
engineers joining Cyclone: process & network topology, tech stack
(backend + frontend + external), backend package layout with
one-line module responsibility map, frontend architecture (route
map, state management, live tail), data model (migrations 0001-0012,
ERD, 7-state claim lifecycle, audit chain), data flow (manual upload,
SFTP polling, auto-match, outbound 837P, ack ingestion), API surface,
cross-cutting concerns (logging, audit, PHI/PII, error model),
operational concerns (startup order, schedulers, health probe,
shutdown), deployment, out-of-scope, and references.
759 lines, 13 top-level sections. Cross-linked from REQUIREMENTS.md
sister-docs block and from README.md Roadmap section.
Tyler
1148a03a43