Pin backend dependencies with pip-compile

Split requirements into requirements.in (direct dependencies with
loose constraints) and requirements.txt (fully pinned lockfile with
hashes, generated by pip-compile). This makes Docker builds
deterministic and prevents unrelated PRs from pulling in breaking
dependency updates.

To update dependencies:
  docker run --rm -v "$(pwd)/backend:/app" -w /app python:3.13-slim-bookworm \
    sh -c "pip install --quiet pip-tools && pip-compile --strip-extras \
    --generate-hashes --output-file requirements.txt requirements.in"

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: ambv

backend/requirements.in

@@ -0,0 +1,14 @@
+fastapi>=0.115.0
+uvicorn[standard]>=0.30.0
+pydantic
+pydantic-settings
+sqlalchemy
+aiosqlite
+asyncpg
+python-multipart
+python-dateutil
+pytest
+pytest-asyncio
+httpx
+authlib>=1.2.0
+sqlparse