chore(MP-30): final docs cleanup, adr consolidation & legacy removal

Summary
- **Documentation Cleanup:**
  - ADRs consolidated to `docs/adr/`.
  - C4 diagrams moved to `docs/c4/`.
  - Removed legacy folder `docs/architecture/` and `docs/clients/`.
- **New ADR:** Added `0009-final-kmp-architecture.md` (Accepted).
- **Updates:**
  - `ARCHITECTURE.md` updated with final folder structure (Core Modules).
  - `README.md` links fixed.
- **Verification:**
  - `./gradlew staticAnalysis` -> SUCCESS.
  - Path references checked.

Ref: MP-30

README.md

@@ -57,8 +57,8 @@ Die Hauptdokumentation befindet sich in der **YouTrack Wissensdatenbank**:
 #### Im Repository
 
 - [📖 docs/README.md](docs/README.md) - Übersicht aller Repository-Dokumentation
-- [🏛️ Architecture Decision Records](docs/architecture/adr)
-- [📐 C4-Diagramme](docs/architecture/c4)
+- [🏛️ Architecture Decision Records](docs/adr)
+- [📐 C4-Diagramme](docs/c4)
 - [🛠️ Developer Guides](docs/how-to)
 - [📑 Projekt-Guidelines (Master)](.junie/guidelines/master-guideline.md)
 
@@ -98,7 +98,7 @@ Das System ist in unabhängige Domänen aufgeteilt:
 - **Polyglot Persistence**: PostgreSQL + Redis
 - **Container-First**: Docker & Docker Compose
 
-**Details**: [ADR-0002 Domain-Driven Design](docs/architecture/adr/0002-domain-driven-design-de.md)
+**Details**: [ADR-0002 Domain-Driven Design](docs/adr/0002-domain-driven-design-de.md)
 
 ---