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/config_misc.py

@@ -1,11 +1,13 @@
 from __future__ import annotations
 
 import shlex
+from pathlib import Path
 from typing import Annotated
 
 import structlog
 from fastapi import APIRouter, Query, Request, status
 
+from app.config import get_settings
 from app.dependencies import (
     AuthDep,
     Fail2BanSocketDep,
@@ -36,6 +38,38 @@ log: structlog.stdlib.BoundLogger = structlog.get_logger()
 router: APIRouter = APIRouter(tags=["Config Misc"])
 
 
+def _validate_log_target(value: str) -> None:
+    """Validate that log_target is either a special value or a valid file path.
+
+    Args:
+        value: The log target to validate.
+
+    Raises:
+        ValueError: If the target is not a special value and not in allowed directories.
+    """
+    if value.upper() in ("STDOUT", "STDERR", "SYSLOG"):
+        return
+
+    settings = get_settings()
+    try:
+        resolved_path = Path(value).resolve()
+    except (OSError, RuntimeError) as e:
+        raise ValueError(f"Cannot resolve path {value!r}: {e}") from e
+
+    for allowed_dir in settings.allowed_log_dirs:
+        allowed_path = Path(allowed_dir).resolve()
+        try:
+            resolved_path.relative_to(allowed_path)
+            return
+        except ValueError:
+            continue
+
+    allowed_dirs_str = ", ".join(settings.allowed_log_dirs)
+    raise ValueError(
+        f"Log target {value!r} is outside allowed directories: {allowed_dirs_str}"
+    )
+
+
 @router.get(
     "/global",
     response_model=GlobalConfigResponse,
@@ -82,9 +116,11 @@ async def update_global_config(
         body: Partial update — only non-None fields are written.
 
     Raises:
-        HTTPException: 400 when a set command is rejected.
+        HTTPException: 400 when a set command is rejected or log_target is invalid.
         HTTPException: 502 when fail2ban is unreachable.
     """
+    if body.log_target is not None:
+        _validate_log_target(body.log_target)
     await config_service.update_global_config(socket_path, body)