feat(backend): enhance performance benchmarking with baseline detection and documentation

- Updated `make benchmark-check` in Makefile to detect and handle missing baselines, creating them if not found.
- Added `.benchmarks` directory to `.gitignore` for local baseline exclusions.
- Linked benchmarking documentation in `ARCHITECTURE.md` and added comprehensive `BENCHMARKS.md` guide.

backend/docs/ARCHITECTURE.md

@@ -1169,6 +1169,8 @@ app.add_middleware(
 
 ## Performance Considerations
 
+> 📖 For the full benchmarking guide (how to run, read results, write new benchmarks, and manage baselines), see **[BENCHMARKS.md](BENCHMARKS.md)**.
+
 ### Database Connection Pooling
 
 - Pool size: 20 connections