Add multi-worker detection for APScheduler safety

- Add _check_single_worker_mode() to startup.py that detects and rejects
  multi-worker configurations, raising a clear RuntimeError with instructions
- Set BANGUI_WORKERS=1 as default in Dockerfile.backend
- Document single-worker requirement in compose.prod.yml
- Add 'Deployment Constraints' section to Architekture.md explaining why
  single-worker mode is required and detailing future multi-worker support
- Add '9.1 Background Tasks and Scheduler Architecture' section to
  Backend-Development.md documenting task structure and single-worker requirement
- Add comprehensive test suite (test_startup.py) covering all scenarios:
  allows single worker, rejects multi-worker, validates config format,
  and verifies informative error messages

This fix addresses TASK-002 which identified that in-process APScheduler is
unsafe in multi-worker deployments due to each worker creating independent
scheduler instances, causing duplicate background job execution.

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

backend/app/startup.py

@@ -7,6 +7,7 @@ in ``app.main`` delegates resource creation and task registration here.
 
 from __future__ import annotations
 
+import os
 from contextlib import suppress
 from pathlib import Path
 from typing import TYPE_CHECKING
@@ -32,6 +33,39 @@ if TYPE_CHECKING:
 log: structlog.stdlib.BoundLogger = structlog.get_logger()
 
 
+def _check_single_worker_mode() -> None:
+    """Verify that the application is running with a single worker.
+
+    APScheduler's AsyncIOScheduler is bound to a single asyncio event loop
+    and cannot be safely shared across multiple worker processes. If each
+    worker starts its own scheduler instance, all background jobs execute N
+    times (where N is the number of workers), resulting in duplicate blocklist
+    imports, duplicate ban operations, duplicate history writes, and SQLite
+    lock contention.
+
+    This function detects multi-worker configurations and raises a clear
+    RuntimeError with instructions.
+
+    Raises:
+        RuntimeError: If the app would run with multiple workers.
+    """
+    # Check for explicit worker count env var (convention used in deployment)
+    workers_env = os.environ.get("BANGUI_WORKERS")
+    if workers_env is not None:
+        try:
+            worker_count = int(workers_env)
+            if worker_count > 1:
+                raise RuntimeError(
+                    "BanGUI background scheduler cannot run with multiple workers.\n"
+                    f"BANGUI_WORKERS is set to {worker_count}. Set it to 1 or remove it.\n"
+                    "See Architekture.md § Deployment Constraints for details."
+                )
+        except ValueError as e:
+            raise RuntimeError(
+                f"BANGUI_WORKERS environment variable must be an integer, got: {workers_env}"
+            ) from e
+
+
 async def _ensure_database_schema(database_path: str) -> None:
     """Create the configured runtime database if it does not already exist."""
     db = await open_db(database_path)
@@ -70,6 +104,8 @@ async def startup_shared_resources(
     Returns:
         A tuple of ``(http_session, scheduler)``.
     """
+    _check_single_worker_mode()
+
     db_path: Path = Path(settings.database_path)
     await run_blocking(db_path.parent.mkdir, parents=True, exist_ok=True)