docs: rename DEPLOY-SUBWORKFLOWS.md to ARCHITECTURE.md and rewrite

Restructured as a proper technical architecture document:
- Added Observability section (correlation IDs, structured errors, debugging)
- Reorganized into logical flow: overview, request flow, contracts, internals
- Removed stale rollback/backup references
- Updated all references in README, CLAUDE.md, PROJECT.md, STATE.md

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

README.md

@@ -126,7 +126,7 @@ Docker Engine
 
 The main workflow handles authentication and routing, while domain-specific logic lives in sub-workflows. This modular design simplifies debugging and isolates container operations.
 
-For detailed sub-workflow contracts and technical architecture, see [DEPLOY-SUBWORKFLOWS.md](DEPLOY-SUBWORKFLOWS.md).
+For detailed sub-workflow contracts and technical architecture, see [ARCHITECTURE.md](ARCHITECTURE.md).
 
 ## Configuration
 
@@ -244,7 +244,7 @@ Send `/start` or any unrecognized text to display the persistent keyboard with q
 
 **Fix:** Click "Apply Update" in Unraid's Docker tab. It completes instantly because the image is already pulled — Unraid just recreates the container to sync its tracking.
 
-For more details, see Known Limitations in [DEPLOY-SUBWORKFLOWS.md](DEPLOY-SUBWORKFLOWS.md).
+For more details, see Known Limitations in [ARCHITECTURE.md](ARCHITECTURE.md).
 
 ### Batch operations not working