TASK-016: Validate delete_log_path query parameter with allowlist

- Extract path validation logic into shared helper function in
  backend/app/utils/path_utils.py (validate_log_path)
- Refactor AddLogPathRequest to use the helper function
- Apply the same validation to DELETE /api/config/jails/{name}/logpath
  endpoint by validating the log_path query parameter
- Return HTTP 422 with descriptive error if validation fails
- Add comprehensive unit tests for path validation
- Update Backend-Development.md with usage examples

This prevents path-traversal attacks on the delete_log_path endpoint
by ensuring all log paths are within allowlisted directories.

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

backend/app/models/config.py

@@ -10,6 +10,7 @@ from typing import Literal
 from pydantic import BaseModel, ConfigDict, Field, field_validator
 
 from app.config import get_settings
+from app.utils.path_utils import validate_log_path
 
 DNSMode = Literal["yes", "warn", "no", "raw"]
 LogEncoding = Literal["auto", "ascii", "utf-8", "UTF-8", "latin-1"]
@@ -295,12 +296,9 @@ class AddLogPathRequest(BaseModel):
 
     @field_validator("log_path", mode="after")
     @classmethod
-    def validate_log_path(cls, value: str) -> str:
+    def validate_log_path_field(cls, value: str) -> str:
         """Validate that the log path is within allowed directories.
 
-        Resolves the path to its canonical form (resolving symlinks) and checks
-        that it is relative to one of the allowed log directories from settings.
-
         Args:
             value: The log path to validate.
 
@@ -310,24 +308,7 @@ class AddLogPathRequest(BaseModel):
         Raises:
             ValueError: If the path is outside allowed log directories.
         """
-        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 value
-            except ValueError:
-                continue
-
-        allowed_dirs_str = ", ".join(settings.allowed_log_dirs)
-        raise ValueError(
-            f"Log path {value!r} is outside allowed directories: {allowed_dirs_str}"
-        )
+        return validate_log_path(value)