OSM: Auto-Retry max 3x (30/60/90s), doppelte Tile-Fetches verhindert

Retry-Limit verhindert Endlos-Loop. Delay wächst (30s→60s→90s) damit
mehr Tiles gecacht sein können. Kartenbewegung setzt Counter zurück.
_fetching-Set im Backend verhindert parallele Doppel-Requests pro Tile.
SW by-v407, APP_VER 387

backend/routes/osm.py

@@ -37,6 +37,9 @@ _OVERPASS_HEADERS   = {
 # Referenzen auf laufende Hintergrund-Tasks — verhindert GC vor Abschluss
 _bg_tasks: set = set()
 
+# Tiles die gerade gefetcht werden — verhindert Doppel-Requests
+_fetching: set = set()
+
 OSM_QUERIES = {
     'waste_basket':   '[out:json][timeout:20];node["amenity"="waste_basket"]({bbox});out;',
     'dog_park':       '[out:json][timeout:25];(way["leisure"="dog_park"]({bbox});node["leisure"="dog_park"]({bbox});way["leisure"="park"]["dog"="yes"]({bbox});node["leisure"="park"]["dog"="yes"]({bbox}););out center;',
@@ -122,6 +125,10 @@ def _stale_tiles(poi_type, tiles):
     return stale
 
 async def _fetch_and_store_tile(poi_type, x, y):
+    fkey = f"{poi_type}_{CACHE_ZOOM}_{x}_{y}"
+    if fkey in _fetching:
+        return  # bereits in Arbeit
+    _fetching.add(fkey)
     key  = f"{CACHE_ZOOM}_{x}_{y}"
     s, w, n, e = _tile_to_bbox(x, y, CACHE_ZOOM)
     query = OSM_QUERIES[poi_type].format(bbox=f"{s},{w},{n},{e}")
@@ -129,6 +136,7 @@ async def _fetch_and_store_tile(poi_type, x, y):
         elements = await _fetch_overpass(query)
     except Exception as exc:
         logger.warning(f"Overpass Fehler {poi_type} Tile {key}: {exc}")
+        _fetching.discard(fkey)
         return
     with db() as conn:
         for el in elements:
@@ -159,6 +167,7 @@ async def _fetch_and_store_tile(poi_type, x, y):
             ON CONFLICT(type, tile_key) DO UPDATE SET cached_at=excluded.cached_at
         """, (poi_type, key))
     logger.info(f"OSM Tile {key} ({poi_type}): {len(elements)} POIs gecacht.")
+    _fetching.discard(fkey)
 
 
 # ------------------------------------------------------------------