refactor: Make service dependencies explicit and injectable

Remove hidden cross-service coupling by making dependencies explicit through
dependency injection while maintaining backward compatibility via lazy imports.

Key changes:
- history_service and ban_service: Removed direct module-level imports of
  fail2ban_metadata_service, added optional service parameters to functions
- Added get_fail2ban_metadata_service() provider to dependencies.py
- Updated history router to inject Fail2BanMetadataService dependency
- history_service functions now use lazy imports in fallback paths for
  backward compatibility when service is not explicitly injected
- All test patches updated to use internal _get_fail2ban_db_path() helper
- jail_config_service and jail_service already follow best practices

This pattern prevents circular imports, makes services testable via explicit
mocking, and documents service dependencies clearly.

Fixes: Instructions.md #2 - Hidden cross-service coupling

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

backend/app/services/protocols.py

@@ -372,6 +372,25 @@ class HealthService(Protocol):
         ...
 
 
+@runtime_checkable
+class Fail2BanMetadataService(Protocol):
+    """Protocol for fail2ban runtime metadata resolution and caching."""
+
+    async def get_db_path(self, socket_path: str, *, force_refresh: bool = False) -> str:
+        ...
+
+    def invalidate_db_path(self, socket_path: str) -> None:
+        ...
+
+
+@runtime_checkable
+class HealthProbe(Protocol):
+    """Protocol for health probing functions that check fail2ban availability."""
+
+    async def __call__(self, socket_path: str) -> ServerStatus:
+        ...
+
+
 @runtime_checkable
 class ServerService(Protocol):
     async def get_settings(self, socket_path: str) -> ServerSettingsResponse: