Add test user seed, password validation, and pre-commit security hook

- backend/scripts/seed.py: creates test@example.com on dev startup
- backend/scripts/start_dev.sh: runs migrations + seed + uvicorn --reload
- backend/app/schemas/user.py: password validator (length, case, digit, special char, forbidden words)
- scripts/security_check.py: Docker-based scanner for secrets, dangerous patterns, weak crypto, bandit
- .githooks/pre-commit: runs security_check.py in python:3.12-slim on every commit

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

backend/scripts/start_dev.sh

@@ -0,0 +1,11 @@
+#!/bin/sh
+set -e
+
+echo "[start] running migrations..."
+alembic upgrade head
+
+echo "[start] seeding dev data..."
+python scripts/seed.py
+
+echo "[start] starting uvicorn..."
+exec uvicorn app.main:app --host 0.0.0.0 --port 8000 --reload