refactor(backend): clean up jail service, add error handling service

- Extract jail status/processing to helper functions
- Add error_handling.py service for centralized error handling
- Update config.py with validation and defaults
- Update .env.example with all config options
- Remove obsolete Tasks.md, add Service-Development.md
- Minor fixes across routers and services

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

backend/app/routers/blocklist.py

@@ -433,6 +433,7 @@ async def preview_blocklist(
     source_id: int,
     http_session: HttpSessionDep,
     blocklist_ctx: BlocklistServiceContextDep,
+    settings: SettingsDep,
     _auth: AuthDep,
 ) -> PreviewResponse:
     """Download and preview a sample of a blocklist source.
@@ -455,7 +456,9 @@ async def preview_blocklist(
         raise BlocklistSourceNotFoundError(source_id)
 
     try:
-        domain_result = await blocklist_service.preview_source(source.url, http_session)
+        domain_result = await blocklist_service.preview_source(
+            source.url, http_session, sample_lines=settings.blocklist_preview_max_lines
+        )
         return blocklist_mappers.map_domain_preview_result_to_response(domain_result)
     except ValueError as exc:
         raise BadRequestError(f"Could not fetch blocklist: {exc}") from exc