Add missing migrations for printer nozzle_count and print_hours_offset

These columns were added in 0.1.6 beta but migrations were missing:
- nozzle_count: Integer DEFAULT 1 (for dual-extruder detection)
- print_hours_offset: Float DEFAULT 0.0 (baseline hours adjustment)

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

backend/app/core/database.py

@@ -947,6 +947,18 @@ async def run_migrations(conn):
     except Exception:
         pass
 
+    # Migration: Add nozzle_count column to printers (for dual-extruder detection)
+    try:
+        await conn.execute(text("ALTER TABLE printers ADD COLUMN nozzle_count INTEGER DEFAULT 1"))
+    except Exception:
+        pass
+
+    # Migration: Add print_hours_offset column to printers (baseline hours adjustment)
+    try:
+        await conn.execute(text("ALTER TABLE printers ADD COLUMN print_hours_offset REAL DEFAULT 0.0"))
+    except Exception:
+        pass
+
 
 async def seed_notification_templates():
     """Seed default notification templates if they don't exist."""