Refactor: Make model packages true leaf nodes - remove app-layer dependencies

Models in app/models/ are now pure data classes with no cross-layer dependencies.
This ensures the models layer remains a true leaf node in the dependency graph.

Changes:
- Create app/models/_common.py with shared types (TimeRange, bucket_count, constants)
- Move TimeRange and time-range constants from ban.py to _common.py
- Update history.py, routers, and services to import from _common.py
- Remove imports from app.config and app.utils from config.py models
- Move field validators from models to router layer:
  - Add log_target validation in config_misc router
  - Add log_path validation in jail_config router
- Update test_models.py to reflect validators moved to router layer
- Update documentation (Architekture.md, Backend-Development.md) with model layering rules
- Fix import ordering and type annotations in affected files

Model layering rule: Models may only import from:
✓ Standard library and third-party packages (Pydantic, typing)
✓ Other models in app/models/ (sibling models)
✓ app.models.response (response envelopes)
✗ app.services, app.config, app.utils, or any application layer

Validation requiring app-level state (settings, allowed directories) now happens
at the router or service layer, not in model validators.

Fixes: Models were not true leaf nodes due to circular imports and app-layer dependencies

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

backend/app/routers/dashboard.py

@@ -25,21 +25,21 @@ from app.dependencies import (
     HttpSessionDep,
     ServerStatusDep,
 )
+from app.mappers import (
+    map_domain_ban_trend_to_response,
+    map_domain_bans_by_country_to_response,
+    map_domain_bans_by_jail_to_response,
+    map_domain_dashboard_ban_list_to_response,
+)
+from app.models._common import TimeRange
 from app.models.ban import (
     BanOrigin,
     BansByCountryResponse,
     BansByJailResponse,
     BanTrendResponse,
     DashboardBanListResponse,
-    TimeRange,
 )
 from app.models.server import ServerStatus, ServerStatusResponse
-from app.mappers import (
-    map_domain_dashboard_ban_list_to_response,
-    map_domain_bans_by_country_to_response,
-    map_domain_ban_trend_to_response,
-    map_domain_bans_by_jail_to_response,
-)
 from app.services import ban_service
 from app.utils.constants import DEFAULT_PAGE_SIZE