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.md

@@ -18,7 +18,7 @@ index: ./.pi-map.index.md
 Trust boundary: index routes, map orients, source decides.
 
 ## role
-A self-hosted web-based project management and Git repository platform with OAuth2 authentication, containerized via Docker Compose.
+Infrastructure and deployment configuration package for a self-hosted project management platform with containerized services, SSO integration, and reverse proxy support.
 ## files
 - .env.example | Provides a template of environment variables for configuring a Headquarter application with PostgreSQL, Redis, Authentik SSO, and Docker/Traefik deployment
 - .gitignore | Specifies files and directories for Git to ignore across a multi-language project with Python, Node, and custom tooling | dep: Git
@@ -31,7 +31,7 @@ A self-hosted web-based project management and Git repository platform with OAut
 - progress.md | Tracks completed and remaining tasks for a backend-frontend code refactoring project organized in 7 phases
 - swap-pane | Empty file with no functionality
 ## arch
-Multi-tier containerized architecture with separate frontend/API services, PostgreSQL/Redis data layer, Traefik reverse proxy integration, and environment-driven configuration.
+Docker Compose-based microservices architecture with PostgreSQL/Redis data layer, separate API/frontend containers, Traefik edge routing, and environment-driven configuration management.
 ## tags
 docker, redis, git, application, postgresql, compose, traefik, project
 ## symbols