Add manual AMS slot selection for prints (#76)

- ReprintModal: Dropdown to override auto-matched AMS slots
- AddToQueueModal: Collapsible filament mapping section
- Backend: Store ams_mapping in print queue, apply when print starts
- Color names in dropdowns (decoded from Bambu codes or derived from hex)
- Blue ring indicator for manually selected vs auto-matched slots
- Status indicators: green (match), yellow (type only), orange (not found)

Closes #76

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

backend/app/core/database.py

@@ -393,6 +393,12 @@ async def run_migrations(conn):
     except Exception:
         pass
 
+    # Migration: Add ams_mapping column to print_queue for storing filament slot assignments
+    try:
+        await conn.execute(text("ALTER TABLE print_queue ADD COLUMN ams_mapping TEXT"))
+    except Exception:
+        pass
+
 
 async def seed_notification_templates():
     """Seed default notification templates if they don't exist."""