Fix backend dependencies and requirements structure

- Update outdated dependencies (FastAPI 0.115.0, SQLAlchemy 2.0.36, etc.)
- Remove duplicate python-multipart dependency
- Replace psycopg2-binary with psycopg2 for production
- Remove redundant aioredis, use redis package only
- Move development dependencies to requirements-dev.txt
- Update all packages to latest stable versions

🤖 Generated with [Claude Code](https://claude.ai/code)

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

backend/requirements-dev.txt

@@ -0,0 +1,22 @@
+# Development Dependencies
+# Install with: pip install -r requirements-dev.txt
+
+# Include production requirements
+-r requirements.txt
+
+# Testing
+pytest==8.3.4
+pytest-asyncio==0.24.0
+pytest-cov==6.0.0
+pytest-mock==3.14.0
+
+# Code Formatting and Linting
+black==24.10.0
+isort==5.13.2
+mypy==1.14.0
+flake8==7.1.1
+bandit==1.8.0
+
+# Development Tools
+pre-commit==4.0.1
+coverage==7.6.9