chore: pin all Python dependency versions in requirements.txt (#62)

- Pin all direct dependencies to exact versions captured from production
  via `docker exec sba_db_api pip freeze`
- Explicitly pin starlette==0.52.1 (root cause of 2026-03-09 outage)
- Move pytest/pytest-asyncio to new requirements-dev.txt

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

requirements-dev.txt

@@ -0,0 +1,2 @@
+pytest==9.0.2
+pytest-asyncio==1.3.0

requirements.txt

@@ -1,11 +1,10 @@
-fastapi
+fastapi==0.133.0
-uvicorn
+uvicorn==0.41.0
+starlette==0.52.1
 peewee==3.13.3
-python-multipart
+python-multipart==0.0.22
-numpy<2.0.0
+numpy==1.26.4
-pandas
+pandas==3.0.1
-psycopg2-binary>=2.9.0
+psycopg2-binary==2.9.11
-requests
+requests==2.32.5
-redis>=4.5.0
+redis==7.3.0
-pytest>=7.0.0
-pytest-asyncio>=0.21.0