feat(FN-019): draft enhanced architecture.md with all 18 required sections (+4 more)

Commits merged:
- docs(FN-019): complete Step 6 — documentation index, conversation handoff, project brief, and build config
- test(FN-019): complete Step 4 — doc validation tests, project-brief.md, and fix mvp-scope placeholders
- feat(FN-019): complete Step 3 — draft mvp-scope.md with milestones, dependency order, and open questions
- docs(FN-019): fix auth callback flow, Python syntax, dev bypass clarity, add AccessProvider protocol and type stubs
- feat(FN-019): complete Step 2 — draft enhanced architecture.md with all 18 required sections

Files changed:
docs/README.md                  |   11 +-
 docs/architecture.md            | 1185 ++++++++++++++++++++++++++++++++++-----
 docs/conversation-handoff.md    |   68 +++
 docs/mvp-scope.md               |  184 ++++++
 docs/project-brief.md           |   31 +
 package.json                    |    3 +
 tests/docs/__init__.py          |    0
 tests/docs/test_architecture.py |  120 ++++
 8 files changed, 1444 insertions(+), 158 deletions(-)

Fusion-Task-Id: FN-019

docs/README.md

@@ -4,12 +4,17 @@ This directory contains architecture, development, and deployment documentation
 
 ## Index
 
-- [Architecture](architecture.md) — System architecture, stack decisions, and MVP phases *(FN-001)*
+- [Architecture](architecture.md) — Canonical system architecture, domain model, provider contracts, and security boundaries *(FN-019)*
+- [MVP Scope](mvp-scope.md) — MVP boundaries, user journeys, milestones, and dependency order *(FN-019)*
+- [Project Brief](project-brief.md) — High-level product context: what, why, who, and confirmed stack *(FN-019)*
 - [Development](development.md) — Local setup, prerequisites, and day-to-day commands *(FN-002)*
-- [Deployment](deployment.md) — Deployment assumptions, Portainer/Traefik skeleton, and follow-up scope *(FN-002)*
+- [Deployment](deployment.md) — Deployment assumptions, Portainer/Traefik skeleton, and operator guide *(FN-002)*
+
+## Reference
+
+- [Conversation Handoff](conversation-handoff.md) — Key architectural decisions, assumptions, and open loops from planning *(FN-019)*
 
 ## Quick Links
 
 - [Root README](../README.md)
 - [Deploy Skeleton](../deploy/README.md)
-- [Project Brief](project-brief.md) — Original product brief and confirmed stack *(FN-001)*