docs: reorganize migration docs into dedicated subfolder

- Create docs/development/migration/ directory
- Move database-migrations.md to migration subfolder
- Move svc-006-migration-plan.md to migration subfolder
- Update all cross-references in:
  - mkdocs.yml nav configuration
  - docs/index.md
  - docs/architecture/overview.md
  - docs/backend/overview.md
  - docs/development/contributing.md
  - docs/development/troubleshooting.md
  - docs/getting-started/database-setup.md

This separates migration plans from core documentation.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

docs/index.md

@@ -179,7 +179,7 @@ platform.com/admin                 → Admin Interface
 
     Alembic migration guide
 
-    [:octicons-arrow-right-24: Migrations](development/database-migrations.md)
+    [:octicons-arrow-right-24: Migrations](development/migration/database-migrations.md)
 
 -   :material-electron-framework:{ .lg .middle } __PyCharm Setup__
 
@@ -324,7 +324,7 @@ When running the application locally:
 
 3. **Start developing**
    - [Backend Development](backend/overview.md)
-   - [Database Migrations](development/database-migrations.md)
+   - [Database Migrations](development/migration/database-migrations.md)
    - [Testing Guide](testing/testing-guide.md)
 
 ### For Operations