fix(scheduler): prevent duplicate slot assignment from concurrent pipeline runs

Two bugs caused multiple articles to land on the same publish slot:

1. main.py: asyncio.create_task() returned immediately, allowing a second
   pipeline trigger (N8N + Telegram /run or two N8N calls) to start a
   second concurrent run. Added asyncio.Lock (_pipeline_lock) so any
   second trigger while the pipeline is running is rejected immediately.

2. scheduler.py: reserve_publish_slot() read the list of occupied slots
   and wrote the new slot in two separate DB connections. Concurrent threads
   could both see the same "free" slot before either committed its write.
   Fixed by wrapping the entire read-find-write cycle in a threading.Lock
   (_slot_lock) and a single DB connection, so the slot check and the
   slot assignment are atomic.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/main.py

@@ -1,8 +1,10 @@
+import asyncio
 from contextlib import asynccontextmanager
 import csv
 from datetime import datetime, timezone
 import io
 import json
+import logging
 from pathlib import Path
 
 from fastapi import Depends, FastAPI, HTTPException, Request, Response, status
@@ -637,20 +639,26 @@ def _require_api_key(request: Request) -> None:
         raise HTTPException(status_code=status.HTTP_401_UNAUTHORIZED, detail="Ungültiger API-Key")
 
 
+_pipeline_lock = asyncio.Lock()
+
+
 @app.post("/api/n8n/pipeline")
 async def api_n8n_pipeline(request: Request) -> dict:
     """Trigger the full auto pipeline in background. Returns immediately.
     Called by N8N (2x/day or on demand). Results arrive via Telegram."""
     _require_api_key(request)
-    import asyncio
-    import logging
+
+    if _pipeline_lock.locked():
+        logging.getLogger(__name__).warning("Pipeline bereits aktiv – Trigger ignoriert")
+        return {"ok": False, "message": "Pipeline läuft bereits – Trigger ignoriert"}
 
     async def _run():
-        loop = asyncio.get_event_loop()
-        try:
-            await loop.run_in_executor(None, lambda: run_auto_pipeline(trigger="n8n"))
-        except Exception as exc:
-            logging.getLogger(__name__).error("Background pipeline error: %s", exc)
+        async with _pipeline_lock:
+            loop = asyncio.get_event_loop()
+            try:
+                await loop.run_in_executor(None, lambda: run_auto_pipeline(trigger="n8n"))
+            except Exception as exc:
+                logging.getLogger(__name__).error("Background pipeline error: %s", exc)
 
     asyncio.create_task(_run())
     return {"ok": True, "message": "Pipeline gestartet – Ergebnisse kommen per Telegram"}