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

.pi-map.index.md

@@ -16,7 +16,7 @@ dir: .
 Trust boundary: index routes, map orients, source decides.
 
 ## role
-A self-hosted project management and git repository platform with OAuth2 authentication, providing containerized deployment via Docker Compose.
+Infrastructure and deployment configuration package for a self-hosted project management platform with containerized services, SSO integration, and reverse proxy support.
 ## parent
 -
 ## children