fix: surface real workspace creation errors instead of generic 409

The create-workspace endpoints caught every exception and returned 409
"Workspace name already exists", hiding the actual failure (e.g. git
clone errors, remote URL problems, or filesystem issues).

- Distinguish ValueError -> 400, unexpected exceptions -> 500 with detail
- Preserve HTTPException re-raise for existing FastAPI error paths

Quality gates:
- pytest tests/unit: 219 passed
- mypy: clean on changed files

apps/.pi-map.md

@@ -4,10 +4,10 @@ dir: apps
 index: apps/.pi-map.index.md
 
 ## role
-Contains the main executable applications and entry points for the project
+Contains the main executable application entry points and top-level app configurations for the project.
 ## files
 ## arch
-Modular application architecture with separate deployable units, likely following a microservices or service-oriented pattern where each subdirectory represents an independent application with its own configuration, dependencies, and lifecycle
+Monorepo-style directory hosting multiple distinct application targets (e.g., web, mobile, desktop, CLI) that consume shared libraries/packages, typically with per-app configuration, routing, and platform-specific bootstrapping.
 ## tags
 -
 ## symbols