perf: add database indexes for better query performance

Indexes added:
- items.artikel - frequently searched/filtered
- items(artikel, groesse) - unique article/size lookup
- orders.done - order completion filter
- orders.canceled - order cancellation filter
- orders(done, canceled) - combined status filter for "open orders"
- ausbuchungen.item_id - foreign key for JOINs

All indexes use IF NOT EXISTS for idempotent execution.
This improves performance for:
- Article search/filtering in admin interface
- Order status filtering
- Stock movement queries

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

wawi/app.py

@@ -160,6 +160,7 @@ def init_db() -> None:
     ensure_price_column(db)
     ensure_image_column(db)
     ensure_orders_columns(db)
+    ensure_indexes(db)
     ensure_admin_user(db)
 
 
@@ -200,6 +201,26 @@ def ensure_orders_columns(db: sqlite3.Connection) -> None:
     db.commit()
 
 
+def ensure_indexes(db: sqlite3.Connection) -> None:
+    """Erstellt Indizes für bessere Query-Performance (idempotent)."""
+    # Index für items.artikel (häufig gesucht/gefiltert)
+    db.execute("CREATE INDEX IF NOT EXISTS idx_items_artikel ON items(artikel)")
+
+    # Index für items(artikel, groesse) für eindeutige Zuordnung
+    db.execute("CREATE INDEX IF NOT EXISTS idx_items_artikel_groesse ON items(artikel, groesse)")
+
+    # Index für orders.done und orders.canceled (Filter "offene Bestellungen")
+    db.execute("CREATE INDEX IF NOT EXISTS idx_orders_done ON orders(done)")
+    db.execute("CREATE INDEX IF NOT EXISTS idx_orders_canceled ON orders(canceled)")
+    db.execute("CREATE INDEX IF NOT EXISTS idx_orders_status ON orders(done, canceled)")
+
+    # Index für ausbuchungen.item_id (Foreign Key, JOINs)
+    db.execute("CREATE INDEX IF NOT EXISTS idx_ausbuchungen_item_id ON ausbuchungen(item_id)")
+
+    db.commit()
+    logger.info("Datenbank-Indizes überprüft/erstellt")
+
+
 @app.before_request
 def ensure_db() -> None:
     """Stellt sicher, dass DB/Schema einmal pro Worker initialisiert ist."""