docs: Define explicit DI container strategy for backend service graph

- Add comprehensive 'Dependency Wiring and Service Composition' section to
  Architekture.md (§ 2.3) documenting:
  * The lightweight FastAPI Depends() pattern used as composition root
  * Service composition through explicit parameter passing
  * Service context dependencies pattern (SessionServiceContext, etc.)
  * Repository boundary enforcement
  * Lifecycle and scope management
  * Checklist for adding new services

- Update Backend-Development.md to reference the new Architecture section
  from the 'Dependency Layering' section

- Enhance dependencies.py module docstring with clear explanation of:
  * Composition root pattern
  * Explicit over implicit principles
  * Service context dependencies
  * Repository boundary enforcement

This resolves issue #39 by providing clear guidance on dependency wiring
without over-engineering. The pattern uses FastAPI's built-in Depends()
framework and avoids heavyweight container libraries, keeping the solution
lightweight and maintainable.

Fixes: #39

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Docs/Backend-Development.md

@@ -312,7 +312,9 @@ async def list_jails(service: JailService = Depends()) -> JailListResponse:
 
 ### Dependency Layering: Enforcing the Repository Boundary
 
-The **repository boundary** separates database-aware code from application logic. This is enforced through dependency injection:
+The **repository boundary** separates database-aware code from application logic. This is enforced through dependency injection.
+
+For a complete overview of BanGUI's DI pattern, including the composition root, service wiring, and lifecycle management, see [Architekture.md § 2.3 Dependency Wiring and Service Composition](Architekture.md#23-dependency-wiring-and-service-composition).
 
 | Layer | Responsibilities | Dependencies |
 |---|---|---|